Add initial implementation of RDF Patch parser.

[recalcitrantsupplant]

Summary of changes

Introduces support for parsing RDF Patch files, aligned to the specification here: https://afs.github.io/rdf-delta/rdf-patch.html

Supports:

• Adding and Deleting triples and quads from a Dataset.
• Adding and Deleting prefixes from a Dataset.
• blank node ID preservation from input documents. That is, blank nodes are NOT document scoped when parsing patches.
  blank nodes can be specified either using the format:
  • _:identifier
  • <_:identifier>
    both will be parsed as blank nodes and their identifier from the document they are parsed from preserved.

Known limitations:

• No support yet for Patch logs (a series of Patch files)

Checklist

• Checked that there aren't other open pull requests for
  the same change.
• Checked that all tests and type checking passes.
• If the change adds new features or changes the RDFLib public API:
  • Created an issue to discuss the change and get in-principle agreement. Discussed with a number of maintainers
    TODO:
  • Considered adding an example in ./examples.
  • Considered adding additional documentation.

[coveralls]

coverage: 90.617% (+0.01%) from 90.605%
when pulling 7262244 on recalcitrantsupplant:david/rdf-patch-parser
into cb5d058 on RDFLib:main.

[ashleysommer]

@edmondchuc Merging is blocked on your pending review. Can you please re-review and approve to so we can merge it.

[edmondchuc]

And can remove these now after the changes in 0e75969?

Probably doesn't affect anything since it defaults to False in the super class.

[recalcitrantsupplant]

Yeah I just mirrored the other parsers here. Could there be use cases where someone wants to skolemize when parsing?