Make mypy stricter

[windymilla]

Add --disallow-untyped-defs option to mypy run.

Without this, it doesn't catch
def highlight_aligncol_in_viewport(viewport: Text):
which doesn't have a return type declared.

[tangledhelix]

Looks good to me - I had believed this check was already in place! I didn't test it myself (Github Actions will do that for us).

[windymilla]

Thanks @tangledhelix - yes, I was surprised when I ran my "check everything" script (where I do have that flag set) and it reported an error in master. I didn't add it originally to Github Actions because I was unsure how much of a pain it would be, and so just left the settings at the defaults.