remove python 3.7 reference from CONTRIBUTING.md

CONTRIBUTING.md

@@ -156,7 +156,7 @@ If you add a new feature, please also include appropriate tests to verify its in
 functionality. We try to keep our code coverage close to 100%.
 
 It is possible to limit the scope of testing to specific environments and files. For example, to
-only test event related functionality using the Python 3.7 environment use:
+only test event related functionality using the Python 3.8 environment use:
 ```bash
 tox -e py38 tests/events
 ```