allow importlib-metadata 6.x

[reaperhulk]

I'm not actually sure if this works, but the 3.7 CI jobs should hopefully show whether this is safe 😄

[codecov-commenter]

Codecov Report

Patch and project coverage have no change.

Comparison is base (da2d660) 81.29% compared to head (a857a4c) 81.29%.

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Additional details and impacted files

```diff @@ Coverage Diff @@ ## develop #424 +/- ## ======================================== Coverage 81.29% 81.29% ======================================== Files 10 10 Lines 770 770 ======================================== Hits 626 626 Misses 144 144 ``` Help us with your feedback. Take ten seconds to tell us [how you rate us](https://about.codecov.io/nps?utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=Andrey+Kislyuk). Have a feature suggestion? [Share it here.](https://app.codecov.io/gh/feedback/?utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=Andrey+Kislyuk)

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[kislyuk]

Thanks. I'm seeing an issue in Python 3.7 CI, but I'm not certain that it's related to importlib-metadata. Need to investigate.

[reaperhulk]

Yeah, it appears there's a failure that also occurs in 5.x (as you filed an issue about it in #420)

[kislyuk]

OK, Python 3.7 is EOL soon anyway so I've marked these tests as skip as well.

Released in v3.0.6. If anyone sees any issues with importlib_metadata > 4 on Python 3.7, please let me know and we can troubleshoot.