Set minimum Python version to 3.7 in README.md (#174)

The latest version of ``trio-websocket`` just bumped the minimum version
of Python to ``3.7``, but the README.md still shows an outdated version.

Author: mdmintz

README.md

@@ -27,7 +27,7 @@ same port is straightforward.  There has yet to be a performance comparison.
 
 ## Installation
 
-This library requires Python 3.6 or greater. To install from PyPI:
+This library requires Python 3.7 or greater. To install from PyPI:
 
     pip install trio-websocket