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Pants

Oh huh...
```"write_files" of "RuleRunner" has incompatible type "Dict[str, str]"; expected
"Mapping[Union[str, PurePath], Union[str, bytes]]"  [arg-type]```
<https://github.com/python/mypy/issues/6001>
&gt; This is because Mapping is invariant in its key type. See <http://mypy.readthedocs.io/en/latest/common_issues.html#invariance-vs-covariance|mypy.readthedocs.io/en/latest/common_issues.html#invariance-vs-covariance>.
Which has turned into a PR to `typing`: <https://github.com/python/typing/issues/445>

*So TL;DR: Don't use `Union` in a `Mapping`'s key type.*