chore: Minimum Python 3.9; Splits optional dependencies

Closes #47
Closes #48

+ Bumps minimum version to Python 3.9
+ Splits optional dependencies

.github/workflows/tests.yaml

@@ -22,7 +22,7 @@ jobs:
       - name: Upgrade pip and install test dependencies
         run: |
           pip install --upgrade pip
-          pip install -e .[dev]
+          pip install -e .[dev,tests]
       - name: Test with pytest
         run: |
           pytest --cov=topofileformats -x