[setup] Pin rdflib version

The latest rdflib version (>5.0.0) brought major breaking changes.
Until these changes have been introduced in the odml library, the
rdflib version will be pinned to 5.0.0 to ensure a full working
library on pip install.

Author: mpsonntag

requirements-test.txt

@@ -1,3 +1,3 @@
 pytest
-owlrl
+owlrl==5.2.3
 requests

setup.py

@@ -31,9 +31,11 @@
 
 # pyparsing needs to be pinned to 2.4.7 for the time being. setup install fetches a pre-release
 # package that currently results in issues with the rdflib library.
-install_req = ["lxml", "pyyaml>=5.1", "rdflib", "docopt", "pathlib", "pyparsing==2.4.7"]
+install_req = ["lxml", "pyyaml>=5.1", "rdflib==5.0.0", "docopt", "pathlib", "pyparsing==2.4.7"]
 
-tests_req = ["pytest", "owlrl", "requests"]
+# owlrl depends on rdflib - the pinned version should be removed once the version pin has also been
+# removed from rdflib.
+tests_req = ["pytest", "owlrl==5.2.3", "requests"]
 
 setup(
     name='odML',