Simplify Python version selection for latest stable

[tkf]

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[codecov-commenter]

Codecov Report

Merging #933 (77d8ee0) into master (1e4a97f) will increase coverage by 0.16%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master     #933      +/-   ##
==========================================
+ Coverage   67.97%   68.14%   +0.16%     
==========================================
  Files          20       20              
  Lines        2033     2012      -21     
==========================================
- Hits         1382     1371      -11     
+ Misses        651      641      -10     

Flag	Coverage Δ
unittests	68.14% <ø> (+0.16%)	⬆️

Flags with carried forward coverage won't be shown. Click here to find out more.

Impacted Files	Coverage Δ
src/pyinit.jl	82.65% <0.00%> (-1.03%)	⬇️
src/PyCall.jl	70.39% <0.00%> (-0.31%)	⬇️
src/startup.jl	53.12% <0.00%> (+4.06%)	⬆️

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