Fix IsSquareMatrix warning in GAP >= 4.16

[fingolfin]

Fixes #26

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 44.75%. Comparing base (3ad26e0) to head (d02057f).
⚠️ Report is 2 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff             @@
##           master      #27      +/-   ##
==========================================
+ Coverage   44.66%   44.75%   +0.09%     
==========================================
  Files          27       27              
  Lines        2653     2652       -1     
==========================================
+ Hits         1185     1187       +2     
+ Misses       1468     1465       -3     

Files with missing lines	Coverage Δ
lib/misc.gd	100.00% <100.00%> (ø)
lib/misc.gi	50.00% <100.00%> (+3.33%)	⬆️

... and 2 files with indirect coverage changes

🚀 New features to boost your workflow:

• ❄️ Test Analytics: Detect flaky tests, report on failures, and find test suite problems.