test Python 3.7 with GitHub actions

this updates the tests to match the Python version described in the README

.github/workflows/tests.yml

@@ -14,6 +14,7 @@ jobs:
       matrix:
         config:
         # [Python version, tox env]
+        - ["3.7",   "py37"]
         - ["3.8",   "py38"]
         - ["3.9",   "py39"]
         - ["3.10",  "py310"]