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/clinic/_contextvars.c.h

@@ -0,0 +1,21 @@
+/*[clinic input]
+preserve
+[clinic start generated code]*/
+
+PyDoc_STRVAR(_contextvars_copy_context__doc__,
+"copy_context($module, /)\n"
+"--\n"
+"\n");
+
+#define _CONTEXTVARS_COPY_CONTEXT_METHODDEF    \
+    {"copy_context", (PyCFunction)_contextvars_copy_context, METH_NOARGS, _contextvars_copy_context__doc__},
+
+static PyObject *
+_contextvars_copy_context_impl(PyObject *module);
+
+static PyObject *
+_contextvars_copy_context(PyObject *module, PyObject *Py_UNUSED(ignored))
+{
+    return _contextvars_copy_context_impl(module);
+}
+/*[clinic end generated code: output=26e07024451baf52 input=a9049054013a1b77]*/