fixing a lot of pycostyle warnings in calculus folder

[fchapoton]

📚 Description

This is fixing a lot of pycodestyle warnings in the calculus folder, in particular in desolvers.py.

📝 Checklist

• The title is concise, informative, and self-explanatory.
• The description explains in detail what this PR is about.
• I have linked a relevant issue or discussion.
• I have created tests covering the changes.
• I have updated the documentation accordingly.

⌛ Dependencies

[github-actions]

Documentation preview for this PR (built with commit af62ec2) is ready! 🎉

[mkoeppe]

LGTM.

Just a quick heads-up: The style fixes to import blocks have a high potential of merge conflicts with my PRs regarding modularization. (No need to change anything here, but some restraint would be helpful.)