Add support for Python 3.10, drop EOL 3.5

[hugovk]

Python 3.10 was released on 2021-10-04:

• https://discuss.python.org/t/python-3-10-0-is-now-available/10955

We can also take the opportunity to drop EOL Python 3.5, and upgrade Python syntax for 3.6+.

nose is abandoned and unmaintained, last release was 6.5 years ago (https://pypi.org/project/nose/) and it no longer works with Python 3.10 (nose-devs/nose#1099). So let's switch to pytest.

We also don't need to create universal wheels, they're for supporting Python 2 and 3.

[Byron]

Thanks a million!

Looking at the minimal required version of GitPython, it should be safe to have a patch release to be OK with the current version of it. But what about older ones? Maybe a major version bump would be in order to avoid breakage?

@hugovk What's your opinion on this?

[hugovk]

Most projects bump the major version when dropping a Python version (and I generally do too), and it's quite a good way to flag it.

But some don't. As long as python_requires is set, pip should still install only a version of the library which matches their running Python, so there should be no sudden breakages.

[Byron]

It looks like the version constraints are set to allow smmap to turn into 5.0: smmap<5,>=3.0.1.

Let me merge this PR and release version 5 accordingly: https://pypi.org/project/smmap/5.0.0/