Introduce mypy for static analysis and fix some bugs that mypy revelaed

[vadim2404]

The idea is to add static code analysis for the code to catch bugs earlier and help engineers with type hinting in the IDEs they use.

[breakid]

I've reviewed all of the code changes, and everything looks good to me. The changes are mostly adding type hints, plus some minor optimizations such as using f-strings and converting some elif blocks to guard statements, which are a bit cleaner.

The only logic change I saw was the following from the append() method in the Edge class, which appears to be a bug fix, but which I wanted to highlight in case someone more familiar with the code base knows of any unintended side effects this might cause.

o.reverse = forward if forward else o.reverse -> o.reverse = reverse if reverse else o.reverse

I'm looking forward to these changes being merged!