Adding stubs for pyre_extensions

[mrkmndz]

Because it was decided (in python/typing#636) that CallableParametersVariable (now renamed ParameterSpecification) wasn't quite ready for inclusion in typing_extensions we went ahead and published our implementation under a new PyPI package, pyre_extensions (https://pypi.org/project/pyre-extensions/).
This is the corresponding typeshed stub.

[srittau]

I am happy to include stubs for pyre_extensions in typeshed if you wish, but wouldn't it better to distribute them with pyre_extensions directly using PEP 561?

• Less work for both the pyre and typeshed teams.
• Stubs always match the implementation.

Normally typeshed includes stubs of packages where the maintainers don't want to or can't support type stubs themselves. Although I can't speak for the typing and mypy teams, I believe the inclusion of typing_extensions and mypy_extensions predate PEP 561.

[mrkmndz]

Pyre does not yet support PEP 561 style stub lookup, and there are some specific deployment specific details internally which makes this approach preferable.