Make it explicit Python 3.7 is required

Author: Jeff Schroeder

README.md

@@ -3,6 +3,8 @@ Pyth Client in Python
 
 A Python library to retrieve data from Pyth account structures off the Solana blockchain.
 
+**NOTE**: This library requires Python 3.7 or greater due to `from __future__ import annotations`.
+
 
 Developer Setup
 ---------------

setup.py

@@ -12,4 +12,5 @@
     extras_require={
         'testing': requirements + ['mock', 'pytest', 'pytest-cov', 'pytest-socket'],
     },
+    python_requires='>=3.7.0',
 )