really drop python<=3.7 support

[kloczek]

Filter all code over pyupgrade --py38.

[codecov[bot]]

Codecov Report

Merging #264 (559c1f3) into main (9f19a18) will not change coverage. The diff coverage is n/a.

Additional details and impacted files

```diff @@ Coverage Diff @@ ## main #264 +/- ## ======================================= Coverage 99.51% 99.51% ======================================= Files 6 6 Lines 416 416 Branches 82 82 ======================================= Hits 414 414 Misses 1 1 Partials 1 1 ``` ------ [Continue to review full report in Codecov by Sentry](https://app.codecov.io/gh/samuelcolvin/watchfiles/pull/264?dropdown=coverage&src=pr&el=continue&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=Samuel+Colvin). > **Legend** - [Click here to learn more](https://docs.codecov.io/docs/codecov-delta?utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=Samuel+Colvin) > `Δ = absolute (impact)`, `ø = not affected`, `? = missing data` > Powered by [Codecov](https://app.codecov.io/gh/samuelcolvin/watchfiles/pull/264?dropdown=coverage&src=pr&el=footer&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=Samuel+Colvin). Last update [9f19a18...559c1f3](https://app.codecov.io/gh/samuelcolvin/watchfiles/pull/264?dropdown=coverage&src=pr&el=lastupdated&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=Samuel+Colvin). Read the [comment docs](https://docs.codecov.io/docs/pull-request-comments?utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=Samuel+Colvin).

[samuelcolvin]

these don't seem related to dropping 3.7, but thanks anyway.

[kloczek]

Thank you 👍