Improve read timeout handling

[joshmoore]

Failures like that in https://latest-ci.openmicroscopy.org/jenkins/job/OMERO-plugins-push/214/console are becoming more common:

Traceback (most recent call last):
  File "/home/omero/workspace/OMERO-plugins-push/build/venv/lib64/python3.6/site-packages/requests/adapters.py", line 449, in send
    timeout=timeout
  File "/home/omero/workspace/OMERO-plugins-push/build/venv/lib64/python3.6/site-packages/urllib3/connectionpool.py", line 720, in urlopen
    method, url, error=e, _pool=self, _stacktrace=sys.exc_info()[2]
  File "/home/omero/workspace/OMERO-plugins-push/build/venv/lib64/python3.6/site-packages/urllib3/util/retry.py", line 400, in increment
    raise six.reraise(type(error), error, _stacktrace)
  File "/home/omero/workspace/OMERO-plugins-push/build/venv/lib64/python3.6/site-packages/urllib3/packages/six.py", line 735, in reraise
    raise value
  File "/home/omero/workspace/OMERO-plugins-push/build/venv/lib64/python3.6/site-packages/urllib3/connectionpool.py", line 672, in urlopen
    chunked=chunked,
  File "/home/omero/workspace/OMERO-plugins-push/build/venv/lib64/python3.6/site-packages/urllib3/connectionpool.py", line 423, in _make_request
    self._raise_timeout(err=e, url=url, timeout_value=read_timeout)
  File "/home/omero/workspace/OMERO-plugins-push/build/venv/lib64/python3.6/site-packages/urllib3/connectionpool.py", line 331, in _raise_timeout
    self, url, "Read timed out. (read timeout=%s)" % timeout_value
urllib3.exceptions.ReadTimeoutError: HTTPSConnectionPool(host='api.github.com', port=443): Read timed out. (read timeout=15)

Possibilities:

• up timeout in constructor: https://pygithub.readthedocs.io/en/latest/github.html
• set a retry: https://urllib3.readthedocs.io/en/latest/reference/urllib3.util.html
• slow the number of calls