Fix supported Python versions in README (#803)

README.rst

diff --git a/README.rst b/README.rst
index 1d52106..333d439 100644
--- a/README.rst
+++ b/README.rst
@@ -9,7 +9,7 @@
 modules with side effects.  It's also much faster.
 
 It is `available on PyPI <https://pypi.org/project/pyflakes/>`_
-and it supports all active versions of Python: 3.6+.
+and it supports all active versions of Python: 3.8+.