the action needs relative_files=true in coverage config, even though the coverage report itself gets it right without this.

we can't submit on macOS because the coverage action uses a docker container. so don't bother to gather coverage data there either.

per https://github.com/coverallsapp/github-action/issues/4#issuecomment-727600521

The PyPI CLI doesn't work.

Python 2 with coveralls 1.11 just produces a 422 error.
Python 3 with 2.2 produces an error on a retry after a 422.

  File "/opt/hostedtoolcache/Python/3.7.9/x64/lib/python3.7/site-packages/coveralls/cli.py", line 85, in main
    result = coverallz.wear()
  File "/opt/hostedtoolcache/Python/3.7.9/x64/lib/python3.7/site-packages/coveralls/api.py", line 225, in wear
    self.config['service_job_id'], random.randint(0, sys.maxsize))
KeyError: 'service_job_id'

Also update the 3.5 certificates to something not expired.

Fixes #1692

minor doc fix