gh-129005: Move bytearray to use bytes as a buffer

[cmaloney]

1. Add tp_new to guarantee ob_bytes_head is always set, often to the empty bytes singleton.
2. ob_alloc information is now redundant, added assertion to validate that, would it make sense to deprecate?
3. There's a lot of bytearray code very similar to bytes code, more could likely be just proxied to the bytes now (Ex. construction from an object in __new__). Here just focusing on the swap as that enables optimizations.
4. If bytearray is passed a single-reference bytes it could potentially take "ownership" of it without copying the bytes, for now not implemented.

This enables adding bytearray._detach() which I plan to do in a separate PR.

TODO

• Single byte strings passed to PyBytes_FromStringAndSize returns immortal objects that shouldn't be mutated. Not sure how to best work around that. _io.FileIO.readall() can't run into that case as it never passes in a string to start with, just a size.

Performance

# _io.FileIO.readall of a large file
./python -m test -M8g -uall test_largefile -m test.test_largefile.CLargeFileTest.test_large_read

# _pyio.FileIO.readall of a large file
./python -m test -M8g -uall test_largefile -m test.test_largefile.PyLargeFileTest.test_large_read

On my machine (AMD 64 bit Linux, Optimized build):
_io takes: ~0.791s and uses ~2GB of RAM
_pyio current: ~1.073s and uses ~4GB of RAM
_pyio with bytearray._detach: ~0.887s and uses ~2GB of RAM

Perf checking no major swings in an optimized build: ./python -E -bb -Wd -m test -uall -M32G test_bytes test_capi.test_bytearray -vvv

before: ~1.4s
after: ~1.4s

Previous discussion: https://discuss.python.org/t/add-zero-copy-conversion-of-bytearray-to-bytes-by-providing-bytes/79164

cc: @serhiy-storchaka @vstinner

• Issue: Reduce copies when reading files in pyio, match behavior of _io #129005

[cmaloney]

behavior change: _PyBytes_Resize behaves differently than PyMem_Realloc here. _PyBytes_Resize can change the location of the buffer. That means the address returned from PyByteArray_AS_STRING may change on any resize. Previously it changed only when went from empty <-> non-empty bytearray (empty points to a constant storage).

https://docs.python.org/3/c-api/memory.html#c.PyMem_Realloc

[cmaloney]

Closing while discussion is ongoing, if decide to ship, will make a new github issue for the new feature this implements.