update minimum Python version in README

Author: belm0

README.md

@@ -31,7 +31,7 @@ same port is straightforward.  There has yet to be a performance comparison.
 
 ## Installation
 
-This library requires Python 3.7 or greater. To install from PyPI:
+This library requires Python 3.8 or greater. To install from PyPI:
 
     pip install trio-websocket