Skip `checksum32` tests if `crc32c` is missing

[jakirkham]

If crc32c is not installed, skip the tests in test_checksum32.py.

---

TODO:

• [ ] Unit tests and/or doctests in docstrings
• [ ] Tests pass locally
• [ ] Docstrings and API docs for any new/modified user-facing classes and functions
• [ ] Changes documented in docs/release.rst
• [ ] Docs build locally
• [ ] GitHub Actions CI passes
• [ ] Test coverage to 100% (Codecov passes)

[codecov[bot]]

Codecov Report

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

Project coverage is 94.54%. Comparing base (7523fc6) to head (4b3dc36). Report is 1 commits behind head on main.

:exclamation: There is a different number of reports uploaded between BASE (7523fc6) and HEAD (4b3dc36). Click for more details.

HEAD has 11 uploads less than BASE

| Flag | BASE (7523fc6) | HEAD (4b3dc36) | |------|------|------| ||12|1|

Additional details and impacted files

```diff @@ Coverage Diff @@ ## main #635 +/- ## ========================================== - Coverage 99.81% 94.54% -5.28% ========================================== Files 62 62 Lines 2711 2712 +1 ========================================== - Hits 2706 2564 -142 - Misses 5 148 +143 ``` | [Files with missing lines](https://app.codecov.io/gh/zarr-developers/numcodecs/pull/635?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\_checksum32.py](https://app.codecov.io/gh/zarr-developers/numcodecs/pull/635?src=pr&el=tree&filepath=numcodecs%2Ftests%2Ftest_checksum32.py&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=zarr-developers#diff-bnVtY29kZWNzL3Rlc3RzL3Rlc3RfY2hlY2tzdW0zMi5weQ==) | `100.00% <100.00%> (ø)` | | ... and [9 files with indirect coverage changes](https://app.codecov.io/gh/zarr-developers/numcodecs/pull/635/indirect-changes?src=pr&el=tree-more&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=zarr-developers)