I met the same issue on the google-api-python-client about the pkg_resources exception (
googleapis/python-api-core#27)

pkg_resources.DistributionNotFound: The 'google-api-python-client' distribution was not found and is required by the application

I found the google-auth source and python-api-core has changed. Could it be changed this way accordingly?
googleapis/google-auth-library-python@ff6b52c