Proposal: Ignore vendor/skeleton/rules.py in coverage report.

Although % coverage is arguably a vanity metric, I believe that it is nice to get as close to 100% as possible. I reviewed all of the currently files that are not covered at all.

[1] and [2] should probably have tests.

However, [3] appears to be an "interface", since this is something that can't really be tested, I propose that we omit it from the coverage report using the omit configuration option[4] of coverage.py.