Fixup another Python 3.6 reference.

Author: acolomb

README.rst

@@ -6,7 +6,7 @@ The aim of the project is to support the most common parts of the CiA 301
 standard in a simple Pythonic interface. It is mainly targeted for testing and
 automation tasks rather than a standard compliant master implementation.
 
-The library supports Python 3.6+.
+The library supports Python 3.8+.
 
 
 Features