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]

Codecov Report

All modified and coverable lines are covered by tests ✅

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

❗ 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

@@            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	Coverage Δ
numcodecs/tests/test_checksum32.py	100.00% <100.00%> (ø)

... and 9 files with indirect coverage changes