PR #709 breaks python 3.6.1 compatibility

[noamkremen]

#709
src/nacl/bindings/crypto_generichash.py:14
from typing import NoReturn, TypeVar

NoReturn was introduced in python 3.6.2, on 3.6.1 fails with:
ImportError: cannot import name ‘NoReturn’

[reaperhulk]

Interesting -- what are you running on that has Python 3.6.1 out of curiosity?

[reaperhulk]

The two potential fixes are to drop NoReturn or drop support for < 3.6.2. I've been unable to find a Linux distribution under current support that ships 3.6.0/3.6.1, but I'm interested in your particular case.

[noamkremen]

An internal build environment, via pyenv.
typing_extensions makes NoReturn available in python 3.6.1, if you are willing to introduce it as a dependency.

[noamkremen]

To give more context, my specific path to this error was via paramiko, which will pull in the latest (> 1.0.1) pynacl version by default during installation.
https://github.com/paramiko/paramiko/blob/7714caf79a09dc455a32c6071dd22ba37c399758/setup.py

[yuvalshirav]

This is an issue with PyGithub (https://github.com/PyGithub/PyGithub) on 3.6.1.

[reaperhulk]

@yuvalshirav are you seeing a lot of 3.6.0/3.6.1 users? I'm inclined to just drop NoReturn but am still considering dropping < 3.6.2 if it's sufficiently rare.