Restore SPARQLWrapper backed SPARQLStore from RDFLib 4.2.2

This was removed in 5.0.0 (see
RDFLib/rdflib#744), but it may be useful for
people wanting backwards compatebility, or who relied on particular
SPARQLWrapper features.

In retrospect, this code should always have been in the SPARQLWrapper
project, avoiding the awkward circular rdflib <-> SPARQLWrapper
dependency :)

Now you can install rdflib + sparqlwrapper and do:

```py

>>> import rdflib
>>> g = rdflib.Graph('SPARQLWrapper', identifier='http://dbpedia.org')
>>> g.open('http://dbpedia.org/sparql')
>>> g.value(rdflib.URIRef('http://dbpedia.org/resource/Berlin'), rdflib.RDFS.label)
rdflib.term.Literal(u'Berlin', lang='en')

```

Author: gromgull