Upgrade Python syntax and add python_requires for 3.5+

[hugovk]

Fixes #391.

• Uses https://github.com/asottile/pyupgrade to upgrade Python syntax for Python 3.5+
• Add python_requires='>=3.5' metadata to help pip

[tyarkoni]

Eep, I just opened #392...

Aside from minor differences between PRs, I'm not sure we want to drop 3.5 support yet given that ~20% of people are still on it. Do people have thoughts?

[hugovk]

Ah! I thought Python 3.5 had already been dropped in #372, but that must just be the testing of 3.5.

Python 3.5 is still supported by core Python team until later this year, so it's fine to still keep it.

Although, NEP 29 is something to consider: https://numpy.org/neps/nep-0029-deprecation_policy.html

Let me know, and I'll drop that change!

[coveralls]

Coverage decreased (-5.03%) to 71.324% when pulling 20a9db1 on hugovk:rm-2 into c272971 on tyarkoni:master.

[tyarkoni]

Thanks!