Add some misc typing to zarr.util

[dstansby]

Some more typing - this time some low hanging fruit in zarr.util.

TODO:

• Add unit tests and/or doctests in docstrings
• Add docstrings and API docs for any new/modified user-facing classes and functions
• New/modified features documented in docs/tutorial.rst
• Changes documented in docs/release.rst
• GitHub Actions have all passed
• Test coverage is 100% (Codecov passes)

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 99.99%. Comparing base (6105ef2) to head (c18bc35).

❗ Current head c18bc35 differs from pull request most recent head 6da0dc2. Consider uploading reports for the commit 6da0dc2 to get more accurate results

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #1617   +/-   ##
=======================================
  Coverage   99.98%   99.99%           
=======================================
  Files          38       38           
  Lines       14640    14580   -60     
=======================================
- Hits        14638    14579   -59     
+ Misses          2        1    -1     

Files	Coverage Δ
zarr/util.py	100.00% <100.00%> (ø)

... and 6 files with indirect coverage changes

[d-v-b]

thanks @dstansby, this just needs release notes

[dstansby]

thanks @dstansby, this just needs release notes

👍 done