Check effectiveness of 5.0.0 fix to memory leak in NamespaceManager

[joernhees]

in its current form NamespaceManager has a __cache = {} attrib:
https://github.com/RDFLib/rdflib/blob/master/rdflib/namespace.py#L286

while there is a reset() i am living proof that this can cause days of debugging to find a memory leak... turns out NamespaceManager held refs to all URIs it ever saw... great.

If i find time and no one objects, i'd add cachetools as dependency and use a LRUCache and make its size configurable....

[nicholascar]

This needs testing for 6.0.0 to see if the namespace tidy-up functions added in 5.0.0 have changed this scenario.