Fix invalid escape sequence

[janmayer]

Hello! When using GPy with Python 3.12, there are a lot of warnings like

GPy\mappings\kernel.py:10
   GPy\mappings\kernel.py:10: SyntaxWarning:invalid escape sequence '\m'

GPy\mappings\linear.py:10
   GPy\mappings\linear.py:10: SyntaxWarning:invalid escape sequence '\m'

GPy\mappings\additive.py:8
   GPy\mappings\additive.py:8: SyntaxWarning:invalid escape sequence '\m'

GPy\mappings\compound.py:7
  GPy\mappings\compound.py:7: SyntaxWarning:invalid escape sequence '\m'

GPy\mappings\constant.py:7
   GPy\mappings\constant.py:7: SyntaxWarning:invalid escape sequence '\m'

GPy\models\warped_gp.py:144
  GPy\models\warped_gp.py:144: SyntaxWarning:invalid escape sequence '\m'

and yes, that is mostly coming from docstrings.

I have used ruff to hopefully find and fix all these occurrences by either adding more slashes, or r-Strings.

Note that ruff hints at many more issues, some of which could be autofixed.

[MartinBubel]

Hi @janmayer
thanks for the contribution!
Test ci looks fine. Is your PR final? If yes, I will merge it.

[janmayer]

Hi, thanks for the quick response! I hope I got them all now, feel free to merge when you see fit.

If you have the time, could you run/integrate a formatter and maybe a linter with autofix, like ruff?
I could also do so, but you'd have to verify an even larger amount of brainless changes.

[MartinBubel]

Thanks for the update. Looks fine and I'll merge.

Regarding ruff, looks interesting. I am using flake8 mostly but felt increasingly unhappy with it when working on large files. Since ruff claims higher speed, I will try it out :)

I will prepare a PR and it and probably ask for your review?