fix pep8 E303 in various folders (plot, quadratic forms, etc)

[fchapoton]

📚 Description

This is fixing pep8 warning E303 too many blank lines in several subfolders of src/sage, including plot and quadratic_forms.

📝 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

[github-actions]

Documentation preview for this PR is ready! 🎉
Built with commit: 85b1af3

[codecov-commenter]

Codecov Report

Base: 88.58% // Head: 88.59% // Increases project coverage by +0.00% 🎉

Coverage data is based on head (85b1af3) compared to base (e310936).
Patch coverage: 100.00% of modified lines in pull request are covered.

Additional details and impacted files

@@           Coverage Diff            @@
##           develop   #35132   +/-   ##
========================================
  Coverage    88.58%   88.59%           
========================================
  Files         2140     2140           
  Lines       396961   396961           
========================================
+ Hits        351657   351681   +24     
+ Misses       45304    45280   -24     

Impacted Files	Coverage Δ
src/sage/plot/animate.py	63.14% <ø> (ø)
src/sage/plot/matrix_plot.py	96.18% <ø> (ø)
src/sage/plot/plot.py	86.48% <ø> (ø)
src/sage/plot/plot3d/tri_plot.py	99.07% <ø> (ø)
src/sage/plot/primitive.py	96.87% <ø> (ø)
src/sage/quadratic_forms/constructions.py	88.88% <ø> (ø)
src/sage/quadratic_forms/genera/genus.py	91.45% <ø> (ø)
src/sage/quadratic_forms/quadratic_form.py	90.09% <ø> (ø)
...e/quadratic_forms/quadratic_form__count_local_2.py	100.00% <ø> (ø)
...ratic_forms/quadratic_form__equivalence_testing.py	81.69% <ø> (ø)
... and 42 more

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