gh-128384: Use a context variable for warnings.catch_warnings (gh-130010)

Make `warnings.catch_warnings()` use a context variable for holding
the warning filtering state if the `sys.flags.context_aware_warnings`
flag is set to true.  This makes using the context manager thread-safe in
multi-threaded programs.

Add the `sys.flags.thread_inherit_context` flag.  If true, starting a new
thread with `threading.Thread` will use a copy of the context
from the caller of `Thread.start()`.

Both these flags are set to true by default for the free-threaded build
and false for the default build.

Move the Python implementation of warnings.py into _py_warnings.py.

Make _contextvars a builtin module.

Co-authored-by: Kumar Aditya <kumaraditya@python.org>

Python/stdlib_module_names.h

@@ -64,6 +64,7 @@ static const char* _Py_stdlib_module_names[] = {
 "_posixshmem",
 "_posixsubprocess",
 "_py_abc",
+"_py_warnings",
 "_pydatetime",
 "_pydecimal",
 "_pyio",