gh-146358: Fix warnings.catch_warnings on Free Threading (GH-146374)
catch_warnings now also overrides warnings.showwarning() on Free
Threading to support custom warnings.showwarning().
(cherry picked from commit 0055140b2c)
Co-authored-by: Victor Stinner <vstinner@python.org>
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>
