refactor!: Deprecate which() in favor of shutil.which()

[tony]

No description provided.

[codecov]

Codecov Report

Merging #397 (b443ca2) into master (0438586) will decrease coverage by 0.03%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master     #397      +/-   ##
==========================================
- Coverage   72.91%   72.88%   -0.04%     
==========================================
  Files          26       26              
  Lines        2256     2253       -3     
==========================================
- Hits         1645     1642       -3     
  Misses        611      611              

Impacted Files	Coverage Δ
tests/cmd/test_core.py	100.00% <100.00%> (ø)
tests/projects/test_conftest.py	100.00% <100.00%> (ø)
tests/projects/test_git.py	98.91% <100.00%> (+<0.01%)	⬆️
tests/projects/test_hg.py	95.45% <100.00%> (+0.21%)	⬆️
tests/projects/test_svn.py	96.00% <100.00%> (ø)

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