type-stable PyError constructor

[stevengj]

Closes #1020, closes #1021.

Also incidentally fixes test_build.jl when python is not installed; it should use PyCall.python.

[stevengj]

CI failure on Ubuntu with Python 2.7 is actions/setup-python#543 — this is not available in the latest Ubuntu x64 build.

[codecov-commenter]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 67.03%. Comparing base (9bd0908) to head (f5efd34).
⚠️ Report is 18 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #1022      +/-   ##
==========================================
+ Coverage   67.01%   67.03%   +0.01%     
==========================================
  Files          20       20              
  Lines        2016     2017       +1     
==========================================
+ Hits         1351     1352       +1     
  Misses        665      665              

Flag	Coverage Δ
unittests	67.03% <100.00%> (+0.01%)	⬆️

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