gh-116738: Make _bz2 module thread-safe

[yoney]

Make the attributes in _bz2 module thread-safe on the free-threading build. Attributes (eof, needs_input, unused_data) are now stored atomically or accessed via mutex-protected getters.

Even though the operations are protected by locks, the attributes exposed via PyMemberDef (eof, needs_input, unused_data) should still be updated atomically while holding the lock, or accessed via mutex‑protected getters, because they can be read without acquiring the lock.

This issue was not addressed in #140555. A small addition that repeatedly reads these attributes shows the issue under tsan build.

WARNING: ThreadSanitizer: data race (pid=108285)
  Write of size 1 at 0x7fe4798852d0 by thread T13:
    #0 decompress /workspace/ft_bz2_2_tsan/./Modules/_bz2module.c:537 (_bz2.cpython-315td-x86_64-linux-gnu.so+0x413b) (BuildId: 2102c7cc8b4b2ce3a6fb3e9f1155de897bc09d77)
    #1 _bz2_BZ2Decompressor_decompress_impl /workspace/ft_bz2_2_tsan/./Modules/_bz2module.c:609 (_bz2.cpython-315td-x86_64-linux-gnu.so+0x413b)
    #2 _bz2_BZ2Decompressor_decompress /workspace/ft_bz2_2_tsan/./Modules/clinic/_bz2module.c.h:199 (_bz2.cpython-315td-x86_64-linux-gnu.so+0x413b)
...
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Previous atomic read of size 1 at 0x7fe4798852d0 by thread T11:
    #0 _Py_atomic_load_char_relaxed /workspace/ft_bz2_2_tsan/./Include/cpython/pyatomic_gcc.h:311 (python+0x99f082) (BuildId: 30a2cc7d935ef6048a72a7568dc140a7b80f5953)
    #1 PyMember_GetOne /workspace/ft_bz2_2_tsan/Python/structmember.c:37 (python+0x99f082)
    #2 member_get /workspace/ft_bz2_2_tsan/Objects/descrobject.c:180 (python+0x60f405) (BuildId: 30a2cc7d935ef6048a72a7568dc140a7b80f5953)
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SUMMARY: ThreadSanitizer: data race /workspace/ft_bz2_2_tsan/./Modules/_bz2module.c:537 in decompress

cc: @mpage @colesbury @emmatyping

• Issue: Audit all built-in modules for thread safety #116738