Recent PRs beautify all of Leo using ruff format with a line length of 100

[Edward K. Ream]

Recent PRs have:

- Beautified all of Leo's source code files with ruff format.

- Fully integrated ruff format into Leo.

- Improved several of Leo's script helpers used to check git diffs.

ruff format made most of these changes automatically, but I have carefully curated the changes to retain columnar data.

The first comment of PR #4487 (The PR for Leo 6.8.8) contains a detailed summary of all significant changes made. I won't repeat that summary here.

Blackening Leo's sentinels is a prerequisite for using ruff format as Leo's own beautifier. The more I use ruff format, the happier I am to have retired Leo's legacy beautifier. In retrospect, the old beautifier was an embarrassment.

I ate my own dog food during all the recent work. Doing so was important: I saw numerous opportunities for improving how Leo interacted with ruff format. Imo, all the rough edges are gone.

Was all this work worthwhile? My answer is emphatic "Yes!" For the first time, Leo's code base is formatted uniformly in accordance with standard Python conventions. Furthermore, retiring Leo's legacy formatter will ease the burden on Leo's future devs.

Please keep testing the "devel" branch!

Edward