DOC: Bump min Python to 3.6+ in documentation.md

Author: kernc

pdoc/documentation.md

@@ -351,7 +351,7 @@ modified templates into the `directories` list of the
 
 Compatibility
 -------------
-`pdoc` requires Python 3.5+.
+`pdoc` requires Python 3.6+.
 The last version to support Python 2.x is [pdoc3 0.3.x].
 
 [pdoc3 0.3.x]: https://pypi.org/project/pdoc3/0.3.13/