fixing some E502 (unnecessary backslash) in pyx files

[fchapoton]

📚 Description

This removes most instances of pycodestyle warning

E502 the backslash is redundant between brackets

inside pyx files. There remains only a few in rings/, that could be easily fixed when activiting the check in the linter.

📝 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.
• I have created tests covering the changes.
• I have updated the documentation accordingly.

⌛ Dependencies

[codecov-commenter]

Codecov Report

Base: 88.57% // Head: 88.59% // Increases project coverage by +0.01% 🎉

Coverage data is based on head (436e2ed) compared to base (52a81cb).
Patch has no changes to coverable lines.

❗ Current head 436e2ed differs from pull request most recent head 9e7bf7d. Consider uploading reports for the commit 9e7bf7d to get more accurate results

Additional details and impacted files

@@             Coverage Diff             @@
##           develop   #35206      +/-   ##
===========================================
+ Coverage    88.57%   88.59%   +0.01%     
===========================================
  Files         2140     2140              
  Lines       397273   397273              
===========================================
+ Hits        351891   351969      +78     
+ Misses       45382    45304      -78     

Impacted Files	Coverage Δ
src/sage/modular/local_comp/liftings.py	98.33% <0.00%> (-1.67%)	⬇️
src/sage/groups/generic.py	88.34% <0.00%> (-0.68%)	⬇️
src/sage/modular/modform/numerical.py	94.19% <0.00%> (-0.65%)	⬇️
...sage/geometry/hyperbolic_space/hyperbolic_model.py	88.95% <0.00%> (-0.62%)	⬇️
src/sage/doctest/reporting.py	74.45% <0.00%> (-0.44%)	⬇️
src/sage/graphs/graph_plot.py	84.33% <0.00%> (-0.29%)	⬇️
src/sage/modules/free_module.py	87.93% <0.00%> (-0.07%)	⬇️
src/sage/sandpiles/sandpile.py	90.92% <0.00%> (-0.07%)	⬇️
src/sage/combinat/tableau.py	96.11% <0.00%> (-0.05%)	⬇️
src/sage/graphs/generators/random.py	92.46% <0.00%> (ø)
... and 18 more

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[dcoudert]

You let some \ in src/sage/rings/polynomial/pbori/pbori.pyx that can be removed by using some (...).

[fchapoton]

I have removed one of them, and also some empty lines and unused imports.
Most of the remaining backslashes are used to break long lines starting with if. I feel it would not look nicer by adding parentheses instead. And this is not the goal here.

[github-actions]

Documentation preview for this PR is ready! 🎉
Built with commit: 9e7bf7d

[dcoudert]

LGTM.