Error on warnings when testing

[dstansby]

Before https://github.com/zarr-developers/numcodecs/pull/619, it would be good to convert test warnings to errors, so we can make sure there aren't any warnings being raised that we're not expecting.

[codecov[bot]]

Codecov Report

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

Project coverage is 99.91%. Comparing base (0bb51de) to head (8aed073). Report is 1 commits behind head on main.

Additional details and impacted files

```diff @@ Coverage Diff @@ ## main #620 +/- ## ======================================= Coverage 99.91% 99.91% ======================================= Files 59 59 Lines 2405 2407 +2 ======================================= + Hits 2403 2405 +2 Misses 2 2 ``` | [Files with missing lines](https://app.codecov.io/gh/zarr-developers/numcodecs/pull/620?dropdown=coverage&src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=zarr-developers) | Coverage Δ | | |---|---|---| | [numcodecs/tests/test\_compat.py](https://app.codecov.io/gh/zarr-developers/numcodecs/pull/620?src=pr&el=tree&filepath=numcodecs%2Ftests%2Ftest_compat.py&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=zarr-developers#diff-bnVtY29kZWNzL3Rlc3RzL3Rlc3RfY29tcGF0LnB5) | `100.00% <100.00%> (ø)` | | | [numcodecs/tests/test\_msgpacks.py](https://app.codecov.io/gh/zarr-developers/numcodecs/pull/620?src=pr&el=tree&filepath=numcodecs%2Ftests%2Ftest_msgpacks.py&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=zarr-developers#diff-bnVtY29kZWNzL3Rlc3RzL3Rlc3RfbXNncGFja3MucHk=) | `100.00% <100.00%> (ø)` | |

[dstansby]

pre-commit.ci autofix