Restore pyrdfa

[ghost]

Fix for #1390, “Bringing back RDFa support to RDFLib”.

Tactical decision required:

Follow the original approach and copy the pyRdfa and pyMicrodata sources into the RDFLib source tree
or
Add pyrdfa3==3.5.3 and git+https://github.com/RDFLib/pymicrodata.git#egg=pyMicrodata to requirements.

Also, it opens a bit of a can of worms in that using a wikipedia URL results in:

.../site-packages/urllib3/connectionpool.py:1013:
InsecureRequestWarning: Unverified HTTPS request is being made to host 'en.wikipedia.org'.
Adding certificate verification is strongly advised.
See: https://urllib3.readthedocs.io/en/1.26.x/advanced-usage.html#ssl-warnings

[ghost]

Dunno what's happening with drone, maybe I need to re-do the fork into my repos.

[ghost]

Tactical decision required:

Follow the original approach and copy the pyRdfa and pyMicrodata sources into the RDFLib source tree or Add pyrdfa3==3.5.3 and git+https://github.com/RDFLib/pymicrodata.git#egg=pyMicrodata to requirements.

Alternatively ... add pyrdfa3 to requirements to avoid mypy errors preventing drone build from succeeding.

Long-term, if RDFa capability is to be preserved, pyrdfa3 will need to be brought into mypy scope at some point.