Fail more gracefully when we hit API rate limit

[kdelee]

I saw a test failure that stemmed from this plugin hitting the API rate limit....the plugin should perhaps fail more gracefully or skip the test and raise a warning instead of just failing with a 403. Perhaps the warning could even point the user to the docs for the API limits on github.

Traceback (most recent call last):
  File "/root/.venv/lib/python2.7/site-packages/pytest_github/plugin.py", line 276, in pytest_runtest_setup
    if not issue.is_resolved:
  File "/root/.venv/lib/python2.7/site-packages/pytest_github/plugin.py", line 151, in is_resolved
    if not self.is_closed and not set(self._gh_plugin.completed_labels).intersection(self.labels):
  File "/root/.venv/lib/python2.7/site-packages/pytest_github/plugin.py", line 146, in labels
    return [l.name for l in labels]
  File "/root/.venv/lib/python2.7/site-packages/github3/structs.py", line 76, in __iter__
    json = self._get_json(response)
  File "/root/.venv/lib/python2.7/site-packages/github3/structs.py", line 111, in _get_json
    return self._json(response, 200)
  File "/root/.venv/lib/python2.7/site-packages/github3/models.py", line 156, in _json
    raise exceptions.error_for(response)
ForbiddenError: 403 API rate limit exceeded for user ID <secret>