Refresh the Python FDK dependencies to more recent versions.

[harshpai]

Updated the dependency versions as follows:

pbr==5.4.5
pytest==5.4.3
pytest-asyncio==0.12.0
httptools>=0.1.1
pytest-cov==2.9.0

The version upgrade caused deprecation of imp library and it is now replaced with importlib.

Built and tested successfully by running:

- python3 -m venv .venv && source .venv/bin/activate
- pip install tox
- tox

[reclaro]

LGTM

[riconnon]

Looks good.