fix and activate W391 in pyx files

[fchapoton]

📚 Description

Fixes #34929

Activate the check for pycodestyle W391 in pyx files

📝 Checklist

• I have made sure that the title is self-explanatory and the description concisely explains the PR.
• I have linked an issue or discussion.

⌛ Dependencies

[codecov-commenter]

Codecov Report

Base: 88.60% // Head: 88.59% // Decreases project coverage by -0.01% ⚠️

Coverage data is based on head (71925b9) compared to base (c000c95).
Patch has no changes to coverable lines.

Additional details and impacted files

@@             Coverage Diff             @@
##           develop   #35091      +/-   ##
===========================================
- Coverage    88.60%   88.59%   -0.01%     
===========================================
  Files         2136     2136              
  Lines       396142   396142              
===========================================
- Hits        350990   350972      -18     
- Misses       45152    45170      +18     

Impacted Files	Coverage Δ
src/sage/interfaces/qsieve.py	71.30% <0.00%> (-2.61%)	⬇️
src/sage/graphs/generators/random.py	91.26% <0.00%> (-1.37%)	⬇️
src/sage/modular/hecke/algebra.py	94.65% <0.00%> (-1.07%)	⬇️
src/sage/schemes/elliptic_curves/cardinality.py	86.61% <0.00%> (-0.79%)	⬇️
src/sage/schemes/elliptic_curves/hom_velusqrt.py	94.88% <0.00%> (-0.79%)	⬇️
src/sage/groups/generic.py	88.34% <0.00%> (-0.68%)	⬇️
...sage/geometry/hyperbolic_space/hyperbolic_model.py	88.95% <0.00%> (-0.62%)	⬇️
src/sage/sets/integer_range.py	91.41% <0.00%> (-0.51%)	⬇️
src/sage/cpython/_py2_random.py	76.03% <0.00%> (-0.42%)	⬇️
src/sage/combinat/constellation.py	91.18% <0.00%> (-0.41%)	⬇️
... and 13 more

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[edgarcosta]

LGTM