Conditionally required rdflib-jsonld when py < 3.6

Using environment markers in setup.py:
https://www.python.org/dev/peps/pep-0508/#environment-markers

Author: BryceStevenWilley

requirements.txt

@@ -2,7 +2,9 @@
 # pip install -r requirements.txt
 lxml
 requests
-rdflib>=6.0.1
+rdflib>=6.0.1; python_version>="3.7"
+rdflib<6.0.0; python_version<"3.7"
+rdflib-jsonld<=0.5.0; python_version<"3.7"
 pyrdfa3
 mf2py>=1.1.0
 six>=1.11

setup.py

@@ -38,8 +38,10 @@ def get_version():
     packages=find_packages(exclude=['tests',]),
     package_data={'extruct': ['VERSION']},
     install_requires=['lxml',
-                      'rdflib',
-                      'rdflib-jsonld',
+                      'rdflib<6.0.0;python_version<"3.7"',
+                      'rdflib-jsonld;python_version<"3.7"',
+                      # rdflib 6.x.y (only on 3.7 and up) contains jsonld
+                      'rdflib>=6.0.1;python_version>="3.7"',
                       'pyrdfa3',
                       'mf2py',
                       'w3lib',