Make SPARQLWrapper an extra_requires for now

Per discussion in https://github.com/RDFLib/rdflib/pull/359
make SPARQLWrapper an extra_requires for the 4.1 release
series, enabling distribution packagers to upgrade to 4.1
for core RDFLib functionality while creating the required
SPARQLWrapper package for 4.2.

Signed-off-by: Dan Scott <dan@coffeecode.net>
Merge pull request #368 from dbs/sparqlwrapper_optional

Make SPARQLWrapper an extra_requires for now