Remove unnecessary coding, use a few f-strings

[mwtoews]

These edits were automated using:

pyupgrade --py37-plus `find . -name "*.py*"`

and does:

• Remove unnecessary # -*- coding: utf-8 -*- comment, since PEP 3120 declared UTF-8 as the default source encoding since Python 3.0
• Convert a few f-strings
• Add missing r prefix for a raw string with \ used for math markup

[MuellerSeb]

That is what I like about OSS: people care about it! Thanks for that. One minor comment about the usage of f-strings in cython.

[MuellerSeb]

Looks good to me! Thank you for this cleanup @mwtoews