SCons: Update mypy

[Repiteo]

Implements a handful of minor adjustments all revolving around mypy

• Added a new section to the .gitignore to explicitly handle the mypy cache after running the mypy test script (or mypy in general)
• Added a new value to the mypy config: no_implicit_optional = True. This is because mypy made this their default value in later versions¹, so adding a rule one way or the other clears up ambiguity for mypy installed from other sources (eg: vscode extension)
• Fixes two files that weren't conforming to no_implicit_optional = True. While they could've been left alone if the value was changed to false, all cases of changed arguments were right beside another argument that was already using an explicit optional wrapper; as such, the changes were sensible in-context & just so happened to be the only offending areas in the entire repo to begin with

Footnotes

1. https://github.com/hauntsaninja/no_implicit_optional#whats-going-on ↩

[akien-mga]

Thanks!