Fix tox config

[aucampia]

The current tox config does not work as the dependencies are out of sync
and doctest has troubles with xunit.

This fixes it so that tox works again and makes it easier for other
people to contribute.

Fixes #1312

Proposed Changes

• Don't run tests with xunit
• Use requirements.txt and requirements.dev.txt instead of duplicating requirements.

[coveralls]

Coverage remained the same at 75.523% when pulling 9197e7b on iafork:fix_tox into 8d592a2 on RDFLib:master.

[coveralls]

Coverage remained the same at 75.523% when pulling 9197e7b on iafork:fix_tox into 8d592a2 on RDFLib:master.

[coveralls]

Coverage remained the same at 75.523% when pulling 9197e7b on iafork:fix_tox into 8d592a2 on RDFLib:master.

[coveralls]

Coverage decreased (-0.006%) to 75.517% when pulling 3d42597 on iafork:fix_tox into 8d592a2 on RDFLib:master.

[nicholascar]

Fine by me but please accept my PR to your branch first which updates the tox supported Python versions which should now be 3.6, - 3.9 only. Thanks.

[nicholascar]

Actually, in addition to my Python version update, please remove space between -r and requirements files, as per StackOverflow

[aucampia]

Actually, in addition to my Python version update, please remove space between -r and requirements files, as per StackOverflow

Changed it as requested, thanks for the heads up :)

[nicholascar]

@white-gecko @ashleysommer over to you. Looks good to me