Fix quoting of variables used in Python calls.

[matthewwardrop]

Adding of the variable tracing broke python argument quoting, apologies to all affected (and thanks for reporting @bashtage).

closes: #151

[codecov]

Codecov Report

Patch coverage: 100.00% and no project coverage change.

Comparison is base (26763ff) 100.00% compared to head (0734fe3) 100.00%.
Report is 2 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff            @@
##              main      #154   +/-   ##
=========================================
  Coverage   100.00%   100.00%           
=========================================
  Files           49        49           
  Lines         2704      2713    +9     
=========================================
+ Hits          2704      2713    +9     

Flag	Coverage Δ
unittests	100.00% <100.00%> (ø)

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Files Changed	Coverage Δ
formulaic/materializers/base.py	100.00% <100.00%> (ø)
formulaic/utils/stateful_transforms.py	100.00% <100.00%> (ø)
formulaic/utils/variables.py	100.00% <100.00%> (ø)

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