Allow default argument to be passed as keyword to os._Environ #15193

As of #15085, this code is invalid according to the typeshed stubs:

import os

os.environ.get("foo", default="bar")

Both mypy and ty now disallows this code despite it being valid at runtime.
main.py:3: error: No overload variant of "get" of "Mapping" matches argument types "str", "str" [call-overload]

At the very least I think os._Environ should have an overload to explicitly allow default to be passed as a keyword argument.