Matplotlib 3.7.0 support

[kiwifb]

📚 Description

Matplotlib 3.7.0 introduce some changes to the labelling of objects in subplots (mainly "subplot" is gone). This PR fixes the affected doctests.
There are also some deprecations warnings that can be dealt with by using objects already available in matplotlib 3.6.
Finally one user warning has gone away.

📝 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.

[codecov-commenter]

Codecov Report

Base: 88.60% // Head: 88.59% // Decreases project coverage by -0.02% ⚠️

Coverage data is based on head (577b9ca) compared to base (8f5bbd2).
Patch coverage: 100.00% of modified lines in pull request are covered.

Additional details and impacted files

@@             Coverage Diff             @@
##           develop   #35177      +/-   ##
===========================================
- Coverage    88.60%   88.59%   -0.02%     
===========================================
  Files         2140     2140              
  Lines       397273   397273              
===========================================
- Hits        352004   351958      -46     
- Misses       45269    45315      +46     

Impacted Files	Coverage Δ
src/sage/plot/contour_plot.py	92.23% <ø> (ø)
src/sage/plot/graphics.py	87.44% <ø> (ø)
src/sage/plot/multigraphics.py	85.20% <ø> (ø)
src/sage/plot/plot3d/plot_field3d.py	100.00% <100.00%> (ø)
src/sage/crypto/util.py	91.37% <0.00%> (-5.18%)	⬇️
src/sage/interfaces/qsieve.py	71.30% <0.00%> (-2.61%)	⬇️
src/sage/combinat/combination.py	93.50% <0.00%> (-1.50%)	⬇️
src/sage/combinat/posets/poset_examples.py	87.67% <0.00%> (-1.01%)	⬇️
src/sage/coding/channel.py	97.10% <0.00%> (-0.73%)	⬇️
...sage/geometry/hyperbolic_space/hyperbolic_model.py	88.95% <0.00%> (-0.62%)	⬇️
... and 20 more

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[tornaria]

LGTM, my suggestion is of style. Your choice, in any case let's get this merged soon.

[kiwifb]

I think it is all very good feedback. I only did the minimum job suggested by the warnings. I will adopt it ASAP.

[tornaria]

LGTM, tested with the system matplotlib 3.7.0 and 3.6.3 on void linux.

[kiwifb]

Thanks!

[kiwifb]

Just merged to update to latest beta. No issues that need a new review.

[github-actions]

Documentation preview for this PR is ready! 🎉
Built with commit: 577b9ca