Add typing_extensions to allow usage of ParamSpec in python < 3.10

[baluyotraf]

Fixes #269 using the typing_extensions backport.

[patrick-kidger]

LGTM - merged! Thank you for the improvement. :)

[daskol]

FYI https://mypy.readthedocs.io/en/stable/runtime_troubles.html#using-new-additions-to-the-typing-module

import sys
if sys.version_info < (3, 10):
    from typing_extensions import ParamSpec
else:
    from typing import ParamSpec

[baluyotraf]

Do you want me to make another PR with this way of doing it?

[patrick-kidger]

No strong feelings from me! :)

[daskol]

Maybe next time. 😊 The semantics are different, but the main thing is that there's an import guard.

I was vendoring jaxtyping in Arch and noticed a dependency on typing-extensions. Since Arch is rolling release distro, Python is the latest and typing-extension dependency usually can be omitted. It is quite common mistake to import unconditionally typing_extensions or importlib.metadata.