(chore) type hints for tests

[d-v-b]

Adds type hints for test functions. They nearly all return None.

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

Attention: Patch coverage is 99.24812% with 2 lines in your changes missing coverage. Please review.

Project coverage is 99.71%. Comparing base (8d15c02) to head (4e37337).

Files with missing lines	Patch %	Lines
numcodecs/tests/common.py	94.11%	1 Missing ⚠️
numcodecs/tests/test_shuffle.py	94.73%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #698      +/-   ##
==========================================
- Coverage   99.78%   99.71%   -0.08%     
==========================================
  Files          63       63              
  Lines        2754     2765      +11     
==========================================
+ Hits         2748     2757       +9     
- Misses          6        8       +2     

Files with missing lines	Coverage Δ
numcodecs/tests/test_astype.py	100.00% <100.00%> (ø)
numcodecs/tests/test_base64.py	100.00% <100.00%> (ø)
numcodecs/tests/test_bitround.py	100.00% <100.00%> (ø)
numcodecs/tests/test_blosc.py	100.00% <100.00%> (ø)
numcodecs/tests/test_bz2.py	100.00% <100.00%> (ø)
numcodecs/tests/test_categorize.py	100.00% <100.00%> (ø)
numcodecs/tests/test_checksum32.py	100.00% <100.00%> (ø)
numcodecs/tests/test_compat.py	100.00% <100.00%> (ø)
numcodecs/tests/test_delta.py	100.00% <100.00%> (ø)
numcodecs/tests/test_entrypoints.py	100.00% <100.00%> (ø)
... and 25 more

[dstansby]

welcome to the typing rabbit hole/warren 😬