Run mypy and pyright as separate jobs on a single Python version

[victorlin]

Description of proposed changes

Previously, mypy was run under pytest. This was easier to set up, but we've since realized that it's not necessary or practical to run mypy on various Python versions in CI. Instead, it should be run on the minimum supported Python version to ensure types are usable by all Python versions that Augur is compatible with.

Related issue(s)

• Closes https://github.com/nextstrain/augur/issues/1516

Checklist

• [x] mypy and pyright outputs look good
• [x] Checks pass
• [x] If making user-facing changes, add a message in CHANGES.md summarizing the changes in this PR

[codecov[bot]]

Codecov Report

All modified and coverable lines are covered by tests :white_check_mark:

Project coverage is 69.54%. Comparing base (4264701) to head (ceab271). Report is 2 commits behind head on master.

Additional details and impacted files

```diff @@ Coverage Diff @@ ## master #1517 +/- ## ========================================== - Coverage 69.57% 69.54% -0.03% ========================================== Files 73 73 Lines 7792 7786 -6 Branches 1906 1904 -2 ========================================== - Hits 5421 5415 -6 Misses 2086 2086 Partials 285 285 ```

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