[setup.py] Support package installation Python<3.7

The rdflib > 6.0.0 requires Python 3.7. The introduced changes
still support the odml package installation with a Python version
< 3.7 and an rdflib version pinned to 5.0.0.

Author: mpsonntag

setup.py

@@ -35,6 +35,15 @@
 # owlrl depends on rdflib; update any changes in requirements-test.txt as well.
 tests_req = ["owlrl", "pytest", "requests"]
 
+# Keep support for for Python versions below 3.7; relevant for the
+# rdflib usage; rdflib >= 6 does not support Python versions below 3.7.
+if _python_version.minor <= 6:
+    # pyparsing needs to be pinned to 2.4.7 due to issues with the rdflib 5.0.0 library.
+    install_req = ["docopt", "lxml", "pathlib", "pyyaml>=5.1", "rdflib==5.0.0", "pyparsing==2.4.7"]
+
+    # owlrl depends on rdflib and needs to be pinned to a corresponding version.
+    tests_req = ["owlrl==5.2.3", "pytest", "requests"]
+
 setup(
     name='odML',
     version=VERSION,