Change minimum Python requirement to 3.7

We require 3.7 because we use the new dataclasses module and because apparently aiohttp has some incompatibility with Python 3.6.

Signed-off-by: Stan Janssen <stan.janssen@elaad.nl>

docs/index.rst

@@ -42,7 +42,7 @@ Library Installation
 
    $ pip install openleadr
 
-OpenLEADR is compatible with Python 3.6+
+OpenLEADR is compatible with Python 3.7 and higher.
 
 Getting Started
 ===============