feat(runfiles): Added type hints to @rules_python//python/runfiles

[UebelAndre]

This change adds mypy types directly to the rules_python.python.runfiles library as well as regression testing to CI.

[aignas]

For me this looks good, but I'd like to wait for @rickeylev to have a look at this since this would constrain the runfiles library to Python 3.7 and I don't know if this is something we want to do now.

EDIT: According to https://devguide.python.org/versions/, it is already EOL, so this concern may not be applicable and we can move forward to the better future with type annotations.

[UebelAndre]

For me this looks good, but I'd like to wait for @rickeylev to have a look at this since this would constrain the runfiles library to Python 3.7 and I don't know if this is something we want to do now.

EDIT: According to https://devguide.python.org/versions/, it is already EOL, so this concern may not be applicable and we can move forward to the better future with type annotations.

For what it’s worth, 3.7 was chosen because it’s EOL and I thought it would be a sufficiently low bar.

[aignas]

LGTM, thanks