Match Py3.7's available minor version

Is this what's required?

Author: tartley

.github/workflows/test.yml

@@ -18,7 +18,7 @@ jobs:
           - {python-version: "pypy-2.7", toxenv: "pypy"}
           - {python-version: "pypy-3.8", toxenv: "pypy3"}
           - {python-version: "2.7", toxenv: "py27"}
-          - {python-version: "3.7", toxenv: "py37"}
+          - {python-version: "3.7.17", toxenv: "py37"}
           - {python-version: "3.8", toxenv: "py38"}
           - {python-version: "3.9", toxenv: "py39"}
           - {python-version: "3.10", toxenv: "py310"}