convert to GitHub Actions, fix Python 3.5.1 and add newer Pythons

[terencehonles]

I noticed getting tests through Travis is taking a long time now. I had also fixed Python 3.5.1 in #107 but it looks like there were more adjustments to the requirements.txt to not rely on Travis's built in / pre-installed tools.

GitHub Actions does not support Python 3.5.1, but I put a request for it actions/setup-python#187.

[cclauss]

@terencehonles Please see the comment about Python 3.5.1 in #110

@JukkaL Would it be possible to review and land this PR (or similar) to test on current versions of Python?

[terencehonles]

@cclauss ack.

I dropped Python 3.5.1, @gvanrossum are you suggesting to drop 3.5 too? x-ref: #110 (comment)

[terencehonles]

It looks like GitHub Actions would also need to be enabled in this repo/org. You can see the runs in my fork: https://github.com/terencehonles/pyannotate/actions

[terencehonles]

It looks like typed-ast is broken on Python 3.10 and broke since I opened this PR. I changed Python 3.10 to be an optional check for now: https://github.com/terencehonles/pyannotate/runs/2314171569?check_suite_focus=true#step:5:55

[terencehonles]

Looks like that was already fixed but not yet released: python/typed_ast#158

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