Drop support for Python 3.5

[hugovk]

Fixes #508.

Also upgrades Python syntax with pyupgrade --py36-plus

And removes D302, which is redundant and cannot be run now Python 2 is dropped.

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[samj1912]

I think the first one makes more sense as it also leaves some clues as to why it was deprecated.

[hugovk]

Thanks, updated!