Refactor Travis

[JelleZijlstra]

• Run separate jobs for mypy, self-formatting, flake8, and test runs.
• Don't run flake8 in 3.8 because it is broken (and we can't really expect flake8 to always keep up with 3.8 development).

[JelleZijlstra]

(WIP because I just wrote this code in the web editor and I have no idea whether it will work.)

[coveralls]

Pull Request Test Coverage Report for Build 804

• 0 of 0 changed or added relevant lines in 0 files are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage increased (+0.03%) to 96.094%

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Totals
Change from base Build 801:	0.03%
Covered Lines:	2903
Relevant Lines:	3021

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💛 - Coveralls

[coveralls]

Pull Request Test Coverage Report for Build 808

• 0 of 0 changed or added relevant lines in 0 files are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage increased (+0.03%) to 96.094%

---

Totals
Change from base Build 801:	0.03%
Covered Lines:	2903
Relevant Lines:	3021

---

💛 - Coveralls

[JelleZijlstra]

We could speed up builds slightly by only installing what we need, but Travis is pretty fast already and most of this is probably cached anyway, so I don't think it's really worth doing.