Support Python 3.5 and higher.

See https://github.com/jensengroup/propka-3.1/pull/36#discussion_r429094884

README.md

@@ -45,7 +45,7 @@ from within a virtual environment (e.g., via [virtualenv](https://pypi.org/proje
 
 ## Requirements
 
-* Python 3.1 or higher 
+* Python 3.5 or higher 
 
 ## Getting started
 

setup.py

@@ -56,6 +56,6 @@ See http://propka.org/ for the PROPKA web server.
             ],
         },
       zip_safe=True,
-      python_requires='>=3',
+      python_requires='>=3.5',
       test_suite="tests",
 )