gh-108337: Add pyatomic.h header

This adds a new header that provides atomic operations on common data
types. The intention is that this will be exposed through Python.h,
although that is not the case yet. The only immediate use is in
the test file.

Author: colesbury

Doc/whatsnew/3.13.rst

@@ -828,6 +828,8 @@ Build Changes
 * SQLite 3.15.2 or newer is required to build the :mod:`sqlite3` extension module.
   (Contributed by Erlend Aasland in :gh:`105875`.)
 
+* Building CPython now requires a compiler with support for the C11 atomic
+  library, GCC built-in atomic functions, or MSVC interlocked intrinsics.
 
 C API Changes
 =============