This change updates the rbtools.api.cache module to modernize the

code, getting rid of six, updating documentation, and adding type

annotations.
While I was in here, I also changed the clearcache command a bit to

make the command responsible for printing the success message instead of

the clear_cache method in the cache module. This allows us to use

self.stdout rather than just calling print.

Ran unit tests.
Posted some review requests.
Cleared my cache file.