add project root to sys.path in conf.py
added 'requirements.txt' file.
fix on 'requirements.txt' file
mock out the imports of third-party modules when building the docs (snippet from FAQ on readthedocs).
try to fix building docs on readthedocs...
try to fix building docs on readthedocs (II)
try to build the docs on readthedocs (III)
try to build the docs on readthedocs (IV)
try to build the docs on readthedocs (V)
try to build the docs on readthedocs (VI)
try to build the docs on readthedocs (VII)
try to build the docs on readthedocs (VIII)
try to build the docs on readthedocs (IX)
try to build the docs on readthedocs (X)
try to build the docs on readthedocs (XI)
removes unused stuff.
now the base code conforms to pep8.
now the base code conforms to pep8 (II).
now the code conforms to flake8.