Switch to ruff to handle linting and formatting

Ruff handles black, flake8 and isort in one package, and is way faster.
The isort rules had not been enforced, so this commit includes a lot
of import resorting changes.

I switched to flake8-bugbear and the standard black-compatible line
length of 80 + 10% (so max 88 characters), so some line reflowing is
included too.

Finally, because bugbear rightly points out that `setattr()` is less
performant, I've switched the `__signature__` assigment back to using
a direct assignment with type ignore comment.

.gitignore

@@ -28,6 +28,9 @@ share/python-wheels/
 *.egg
 MANIFEST
 
+# Ruff
+.ruff_cache/
+
 # PyInstaller
 #  Usually these files are written by a python script from a template
 #  before PyInstaller builds the exe, so as to inject date/other infos into it.