Add macOS and Windows CI testing

[yarikoptic]

Extend the test matrix to include macOS and Windows platforms with Python 3.9, while keeping all Python versions on Ubuntu. Use pip to install git-annex cross-platform instead of apt.

🤖 Generated with Claude Code

I have not realized that we do not test across OSes thus potentially creating an issue for usage within datalad. Also related is

• Build for all platforms conda-forge/annexremote-feedstock#57 by @matrss

TODOs:

• make work under Windows? I forgot how the special remotes execution etc is concocted under windows, so we might need tune for testing there.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 96.10%. Comparing base (c75c86c) to head (7bf4b81).
⚠️ Report is 32 commits behind head on master.

Additional details and impacted files

@@           Coverage Diff           @@
##           master     #119   +/-   ##
=======================================
  Coverage   96.10%   96.10%           
=======================================
  Files           2        2           
  Lines         385      385           
=======================================
  Hits          370      370           
  Misses         15       15           

Flag	Coverage Δ
unittests	96.10% <ø> (ø)

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[yarikoptic]

ok, we do not try to run that bash example on windows and that makes it all kosher at least in terms of unittesting the library . I will proceed with merge