fix: compatibility with python < 3.8 (#16)

* ci: run tests in python 3.7 as well

.github/workflows/main.yml

@@ -36,17 +36,19 @@ jobs:
               run: |
                 python setup.py check
                 python setup.py bdist_wheel sdist
-
           - python: "3.10"
             task:
-              name: "Style"
+              name: "Lint"
               run: |
                 black --check .
           - python: "3.10"
             task:
               name: "Test"
               run: pytest --exitfirst --failed-first
-
+          - python: "3.7"
+            task:
+              name: "Test (3.7)"
+              run: pytest --exitfirst --failed-first
     steps:
       - uses: actions/checkout@v3