fix pep8 E303 in folders starting with m

[fchapoton]

📚 Description

fix all pep8 E303 warning in the folders whose name starts with the letter m

purely done with autopep8:

$ autopep8 -i --select=E303 src/sage/m*/*.py
$ autopep8 -i --select=E303 src/sage/m*/*/*.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

[dcoudert]

LGTM.