doc: fix build_docs test on travis and add to github ci Travis CI build failed build_doc due to missing sphinx-tabs. Add the package for travis. Also github was not building docs at all. Add builiding docs.

try:
    # Python 3+
    from ssl import SSLError
except (ImportError, AttributeError):
    # Python 2.5-2.7
    from socket import sslerror as SSLError    # noqa: F401