Add requirement for requests to setup.py

[white-gecko]

The requests package is required by the sparql plugin at several places.

The current issue is reproducible with:

In a new virtualenv

$ pip install git+https://github.com/RDFLib/rdflib@master
$ python3
>>> import rdflib
>>> graph = rdflib.Graph()
>>> graph.query("select * {?s ?p ?o}")
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
  File "/home/natanael/.virtualenvs/rdflib-try/lib/python3.7/site-packages/rdflib/graph.py", line 1073, in query
    result = plugin.get(result, query.Result)
  File "/home/natanael/.virtualenvs/rdflib-try/lib/python3.7/site-packages/rdflib/plugin.py", line 107, in get
    return p.getClass()
  File "/home/me/.virtualenvs/rdflib-try/lib/python3.7/site-packages/rdflib/plugin.py", line 69, in getClass
    module = __import__(self.module_path, globals(), locals(), [""])
  File "/home/me/.virtualenvs/rdflib-try/lib/python3.7/site-packages/rdflib/plugins/sparql/__init__.py", line 37, in <module>
    from .processor import prepareQuery, processUpdate
  File "/home/me/.virtualenvs/rdflib-try/lib/python3.7/site-packages/rdflib/plugins/sparql/processor.py", line 18, in <module>
    from rdflib.plugins.sparql.evaluate import evalQuery
  File "/home/me/.virtualenvs/rdflib-try/lib/python3.7/site-packages/rdflib/plugins/sparql/evaluate.py", line 20, in <module>
    import requests
ModuleNotFoundError: No module named 'requests'
>>>

[coveralls]

Coverage remained the same at 75.758% when pulling 6ca6210 on white-gecko:feature/addRequirementForRequests into 0e5efef on RDFLib:master.

[tgbugs]

This should be in extras because the plugin is not required for core.