upgrade necessary python version in __init__.py

synapse/__init__.py

@@ -21,8 +21,8 @@ import os
 import sys
 
 # Check that we're not running on an unsupported Python version.
-if sys.version_info < (3, 6):
-    print("Synapse requires Python 3.6 or above.")
+if sys.version_info < (3, 7):
+    print("Synapse requires Python 3.7 or above.")
     sys.exit(1)
 
 # Twisted and canonicaljson will fail to import when this file is executed to