Fix transitive_objects/subjects docstrings and signatures

[aucampia]

Fixes #1303

Proposed Changes

• Fix signatures of both methods to use predicate.
• Fix incorrect docstring for transitive_subjects

This is a breaking change, because signature of a user facing method was changed. (breaks in case of using named attributes.)

[coveralls]

Coverage decreased (-0.08%) to 75.444% when pulling 9a64d5e on iafork:fix_docstrings into d4c1eff on RDFLib:master.

[coveralls]

Coverage remained the same at 75.523% when pulling 9a64d5e on iafork:fix_docstrings into d4c1eff on RDFLib:master.

[aucampia]

@white-gecko hope this is what you had in mind here

[white-gecko]

Thank you.

[nicholascar]

Given that test pass there must be none that actually test the transitive_objects or transitive_subjects functions. Pity...

[aucampia]

Given that test pass there must be none that actually test the transitive_objects or transitive_subjects functions. Pity...

Yeah, I looked and I could not find any, will try add some when I have time.