Reformat codebase and enforce it going forward #657

On and off, there have been efforts to keep the formatting in check.

Rough outline:

• set up black to format the python sources
• set up indent or clang-format to format the C sources
• reformat things
• create a lint job that enforces no diff so we don't accidentally drift
• just after the last rebase (and before merge) add .git-blame-ignore-revs so the commit doesn't confuse git blame/Github's blame functions

Afterwards:

• rebase pre-existing PRs on top of 3.5 manually (probably along the lines of "rebase up to the offending commit, run black+git reset to acknowledge the reformat, rebase the rest of the way" for those that won't be easy enough to do by hand)

Quite a few big things have landed recently and there are not that many PRs against main that would need our attention anymore which makes 3.5.0 a sweet spot to finish the job.