GH-84976: Move Lib/datetime.py to Lib/_pydatetime

This breaks the tests, but we are keeping it as a separate commit so
that the move operation and editing of the moved files are separate, for
a cleaner history.

Python/stdlib_module_names.h

@@ -56,6 +56,7 @@ static const char* _Py_stdlib_module_names[] = {
 "_posixshmem",
 "_posixsubprocess",
 "_py_abc",
+"_pydatetime",
 "_pydecimal",
 "_pyio",
 "_pylong",