Restricting rdflib package version to "<7"

The following test cases in prov/tests/qnames.py failed with rdflib.7.0.0:
- test_namespace_inheritance
- test_default_namespace_inheritance
(found in issue #156)

Author: trungdong

requirements.txt

@@ -2,4 +2,4 @@ lxml>=3.3.5
 networkx>=2.0
 python-dateutil>=2.2
 pydot>=1.2.0
-rdflib>=4.2.1
+rdflib>=4.2.1,<7

setup.py

@@ -8,7 +8,7 @@
 
 long_description = (here / "README.rst").read_text(encoding="utf-8")
 
-requirements = ["python-dateutil>=2.2", "networkx>=2.0", "lxml>=3.3.5", "rdflib>=4.2.1"]
+requirements = ["python-dateutil>=2.2", "networkx>=2.0", "lxml>=3.3.5", "rdflib>=4.2.1,<7"]
 
 test_requirements = ["pydot>=1.2.0"]