Ignore deprecation warnings about old-style __init__/__setstate__ constructors in the tests (originally done in #2746)

[YannickJadoul]

Description

As the title says. The test suite contains a few tests to make sure the old-style __init__ (with placement news) still works and interacts correctly with the new, non-deprecated way of doing things. Using a debug build of Python (in #2746), pybind11 shows these deprecation warnings, so let's filter these out to remove some output clutter and make sure we have a higher chance of catching useful warnings.

Suggested changelog entry:

Suppressed some deprecation warnings about old-style ``__init__``/``__setstate__`` in the tests.

[YannickJadoul]

Comment from #2746:

• @YannickJadoul: For reference, this was the old code, without py::exec (and thus without requiring <pybind11/eval.h> to be included in pybind11_tests.h and thus in all tests).

  @bstaletic slightly prefers this [new version], and I think I do to. However, curious to learn what other reviewers think.

  /// Simplified ``with warnigns.catch_warnings()`` wrapper
  template <typename F>
  void ignoreOldStyleInitWarnings(F &&body) {
      auto message = "pybind11-bound class '.+' is using an old-style placement-new '(?:__init__|__setstate__)' which has been deprecated";
      auto category = py::reinterpret_borrow<py::object>(PyExc_FutureWarning);
      auto warnings = py::module_::import("warnings");
      auto context_mgr = warnings.attr("catch_warnings")();
      context_mgr.attr("__enter__")();
      warnings.attr("filterwarnings")("ignore", py::arg("message")=message, py::arg("category")=category);
      body();
      // Exceptions in `body` not handled; see PEP 343 when these would need to be added
      context_mgr.attr("__exit__")(py::none(), py::none(), py::none());
  }

[henryiii]

All test changes to avoid warnings because we are intentionally checking things that warn, looks good to me.

[bstaletic]

Since I helped with the PR, am I allowed to approve?

[YannickJadoul]

Let's merge this, then? If anyone objects against #include <pybind11/eval.h> in pybind11_tests.h, we can still change it!