Do not compare literal with is

[mih]

SyntaxWarning: "is" with a literal. Did you mean "=="?

Closes #526

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Comparison is base (d3e50bb) 92.46% compared to head (7c08113) 92.46%.
Report is 2 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main     #527   +/-   ##
=======================================
  Coverage   92.46%   92.46%           
=======================================
  Files         128      128           
  Lines        9647     9647           
  Branches     1045     1045           
=======================================
  Hits         8920     8920           
  Misses        703      703           
  Partials       24       24           

Files	Coverage Δ
datalad_next/constraints/tests/test_compound.py	100.00% <100.00%> (ø)

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