Address mypy type checking errors

[jfhbrook]

#176 shows that mypy's behavior deviates significantly from pyright, to the point that we should QA against it.

This PR also includes a bunch of typing fixes - mostly for the benefit of mypy, though a few fixes for pyright as well. In the latter case, pyright expects the Self type now. Note that this means I'd have to drop support for 3.8, 3.9 and 3.10 - unless, of course, I define Self = Any.

QA passes, but I need to decide whether or not I want to cut a major release.

[jfhbrook]

Just some notes to self.

[jfhbrook]

mypy thinks iscoroutine will always return True. I don't know why it thinks this.

But also, if I drop the conditional import, I shouldn't need to check for iscoroutine.

[jfhbrook]

I think it was actually that iscoroutine would always be truthy. Getting rid of the conditional import may fix this.

[jfhbrook]

I decided to use Self = Any in the typing, but to release as a major due to other changes:

• Overloads for ee.on and various un-annotated None return types
• Removed conditional import of iscoroutine, initially introduced for Python 3.3 support
• Removed twisted.python.Failure type stub, initially introduced due to typing bugs in old versions of Twisted
• Addition of mypy to dev dependencies - these are treated as true dependencies for a "dev" feature in pypi

Once this is merged, I'll make new PRs which add a true Self type and hide some private members of the pyee.cls module, and I'll cut an issue for removing the dev dependency group.