So annoying how quickly this tooling changes. Couple of notes:

- ops/charm.py: it seems like the new version disallows overriding the
  type in a subclass, so just use a "pyright: ignore" comment to work
  around this. Maybe there's a better way.
- The new Pyright no longer allows a type variable to be defined inside
  a function. Not sure what the rationale for this was, but they have
  to be moved to the top level (away from where they're used!) now.
- Some of the bytes/str/bytearray stuff is annoying. Maybe there are
  better solutions for these types. Oh for Go's io.Reader.
-

Hopefully this this fixes CI.