Add minimal version of six

jakubwasikowski · jakubwasikowski · commit 17c2982fa4ec · 2019-07-17T12:19:29.000+02:00
diff --git a/requirements.txt b/requirements.txt
@@ -7,6 +7,6 @@ requests
 rdflib
 rdflib-jsonld
 mf2py>=1.1.0
-six
+six>=1.11
 w3lib
 html-text
