Refine Python CI

Add lint environment to `deltachat-rpc-client/`
and set line length to 120, same as in `python/`.

Switch from flake8 to ruff.

Fix ruff warnings.

python/doc/conf.py

@@ -11,7 +11,8 @@
 # All configuration values have a default; values that are commented out
 # serve to show the default.
 
-import sys, os
+import sys
+import os
 
 # The version info for the project you're documenting, acts as replacement for
 # |version| and |release|, also used in various other places throughout the