Update SECURITY.md to reflect Python 3.7 support dropoff (#304)

Author: kemzeb

.github/SECURITY.md

@@ -4,8 +4,8 @@
 
 | Version | Supported          |
 | ------- | ------------------ |
-| 3.7 +   | :white_check_mark: |
-| < 3.7   | :x:                |
+| 3.8 +   | :white_check_mark: |
+| < 3.8   | :x:                |
 
 ## Reporting a Vulnerability