rules: use Scope enum instead of constants

[williballenthin]

this PR updates code across capa to prefer to use Scope.FOO instead of FOO_SCOPE. before, we had two ways of doing things. now we have one. preferring to use the enum is better because it lets type checkers reason about the set of values, rather than arbitrary strings.

Checklist

• No CHANGELOG update needed

• No new tests needed

• No documentation update needed

[williballenthin]

we should review the rest of the capa code base for similar constructs that we can simplify.

[mr-tz]

LGTM, thank you!

edit: well, tests disagree... :)

[williballenthin]

i think the test failures are in the base branch, too

[yelhamer]

LGTM! Thanks!

Do you think there would be any confusion/mix-up regarding the Scopes and Scope classes? Should we rename Scopes to maybe RuleScopes?

As for the tests, they are failing due to some result document adjustments I believe... I'll work on that and try to get them green again.

[williballenthin]

i could see that someone might temporarily mix up Scope and Scopes, but mypy should be able to highlight the differences pretty quickly, since they have different shapes. i don't have a strong feeling either way.

[yelhamer]

I see. Let's stick to this (what you proposed in this PR) instead then. I'll merge.