Convert old string substitutions to f-strings in term.py (#2864)

* Replace outdated % substition strings in term.py, improves the performance when stringifying URIRef, BNode, and Literal nodes, also replace other outdated patterns in term.py with some modern equivelents for better performance and compatibility.

* Fixed a very old bug where a Generator Function (a yield function) didn't work as a BNode prefix generator, even though it should have. This was revieled by a newly failing test after the change to better string concatenation in the BNode constructor.

* fix docs generation issue for Generator type in new term.py changes.