* CI: Intel icc/icpc via oneAPI

Add testing for Intel icc/icpc via the oneAPI images.
Intel oneAPI is in a late beta stage, currently shipping
oneAPI beta09 with ICC 20.2.

CI: Skip Interpreter Tests for Intel

Cannot find how to add this, neiter the package `libc6-dev` nor
`intel-oneapi-mkl-devel` help when installed to solve this:
```
-- Looking for C++ include pthread.h
-- Looking for C++ include pthread.h - not found
CMake Error at /__t/cmake/3.18.4/x64/cmake-3.18.4-Linux-x86_64/share/cmake-3.18/Modules/FindPackageHandleStandardArgs.cmake:165 (message):
  Could NOT find Threads (missing: Threads_FOUND)
Call Stack (most recent call first):
  /__t/cmake/3.18.4/x64/cmake-3.18.4-Linux-x86_64/share/cmake-3.18/Modules/FindPackageHandleStandardArgs.cmake:458 (_FPHSA_FAILURE_MESSAGE)
  /__t/cmake/3.18.4/x64/cmake-3.18.4-Linux-x86_64/share/cmake-3.18/Modules/FindThreads.cmake:234 (FIND_PACKAGE_HANDLE_STANDARD_ARGS)
  tests/test_embed/CMakeLists.txt:17 (find_package)
```

CI: libc6-dev from GCC for ICC

CI: Run bare metal for oneAPI

CI: Ubuntu 18.04 for oneAPI

CI: Intel +Catch -Eigen

CI: CMake from Apt (ICC tests)

CI: Replace Intel Py with GCC Py

CI: Intel w/o GCC's Eigen

CI: ICC with verbose make

[Debug] Find core dump

tests: use arg{} instead of arg() for Intel

tests: adding a few more missing {}

fix: sync with @tobiasleibner's branch

fix: try ubuntu 20-04

fix: drop exit 1

docs: Apply suggestions from code review

Co-authored-by: Tobias Leibner <tobias.leibner@googlemail.com>

Workaround for ICC enable_if issues

Another workaround for ICC's enable_if issues

fix error in previous commit

Disable one test for the Intel compiler in C++17 mode

Add back one instance of py::arg().noconvert()

Add NOLINT to fix clang-tidy check

Work around for ICC internal error in PYBIND11_EXPAND_SIDE_EFFECTS in C++17 mode

CI: Intel ICC with C++17

docs: pybind11/numpy.h does not require numpy at build time. (#2720)

This is nice enough to be mentioned explicitly in the docs.

docs: Update warning about Python 3.9.0 UB, now that 3.9.1 has been released (#2719)

Adjusting `type_caster<std::reference_wrapper<T>>` to support const/non-const propagation in `cast_op`. (#2705)

* Allow type_caster of std::reference_wrapper<T> to be the same as a native reference.

Before, both std::reference_wrapper<T> and std::reference_wrapper<const T> would
invoke cast_op<type>. This doesn't allow the type_caster<> specialization for T
to distinguish reference_wrapper types from value types.

After, the type_caster<> specialization invokes cast_op<type&>, which allows
reference_wrapper to behave in the same way as a native reference type.

* Add tests/examples for std::reference_wrapper<const T>

* Add tests which use mutable/immutable variants

This test is a chimera; it blends the pybind11 casters with a custom
pytype implementation that supports immutable and mutable calls.

In order to detect the immutable/mutable state, the cast_op needs
to propagate it, even through e.g. std::reference<const T>

Note: This is still a work in progress; some things are crashing,
which likely means that I have a refcounting bug or something else
missing.

* Add/finish tests that distinguish const& from &

Fixes the bugs in my custom python type implementation,
demonstrate test that requires const& and reference_wrapper<const T>
being treated differently from Non-const.

* Add passing a const to non-const method.

* Demonstrate non-const conversion of reference_wrapper in tests.

Apply formatting presubmit check.

* Fix build errors from presubmit checks.

* Try and fix a few more CI errors

* More CI fixes.

* More CI fixups.

* Try and get PyPy to work.

* Additional minor fixups. Getting close to CI green.

* More ci fixes?

* fix clang-tidy warnings from presubmit

* fix more clang-tidy warnings

* minor comment and consistency cleanups

* PyDECREF -> Py_DECREF

* copy/move constructors

* Resolve codereview comments

* more review comment fixes

* review comments: remove spurious &

* Make the test fail even when the static_assert is commented out.

This expands the test_freezable_type_caster a bit by:
1/ adding accessors .is_immutable and .addr to compare identity
from python.
2/ Changing the default cast_op of the type_caster<> specialization
to return a non-const value. In normal codepaths this is a reasonable
default.
3/ adding roundtrip variants to exercise the by reference, by pointer
and by reference_wrapper in all call paths.  In conjunction with 2/, this
demonstrates the failure case of the existing std::reference_wrpper conversion,
which now loses const in a similar way that happens when using the default cast_op_type<>.

* apply presubmit formatting

* Revert inclusion of test_freezable_type_caster

There's some concern that this test is a bit unwieldly because of the use
of the raw <Python.h> functions. Removing for now.

* Add a test that validates const references propagation.

This test verifies that cast_op may be used to correctly detect
const reference types when used with std::reference_wrapper.

* mend

* Review comments based changes.

1. std::add_lvalue_reference<type> -> type&
2. Simplify the test a little more; we're never returning the ConstRefCaster
type so the class_ definition can be removed.

* formatted files again.

* Move const_ref_caster test to builtin_casters

* Review comments: use cast_op and adjust some comments.

* Simplify ConstRefCasted test

I like this version better as it moves the assertion that matters
back into python.

ci: drop pypy2 linux, PGI 20.7, add Python 10 dev (#2724)

* ci: drop pypy2 linux, add Python 10 dev

* ci: fix mistake

* ci: commented-out PGI 20.11, drop 20.7

fix: regression with installed pybind11 overriding local one (#2716)

* fix: regression with installed pybind11 overriding discovered one

Closes #2709

* docs: wording incorrect

style: remove redundant instance->owned = true (#2723)

which was just before set to True in instance->allocate_layout()

fix: also throw in the move-constructor added by the PYBIND11_OBJECT macro, after the argument has been moved-out (if necessary) (#2701)

Make args_are_all_* ICC workarounds unconditional

Disable test_aligned on Intel ICC

Fix test_aligned on Intel ICC

Skip test_python_alreadyset_in_destructor on Intel ICC

Fix test_aligned again

ICC CI: Downgrade pytest

pytest 6 does not capture the `discard_as_unraisable` stderr and
just writes a warning with its content instead.

* refactor: simpler Intel workaround, suggested by @laramiel

* fix: try version with impl to see if it is easier to compile

* docs: update README for ICC

Co-authored-by: Axel Huebl <axel.huebl@plasma.ninja>
Co-authored-by: Henry Schreiner <henryschreineriii@gmail.com>