Use pip install htmd[dev] in CI

.github/workflows/python-package.yml

@@ -29,8 +29,7 @@ jobs:
     - name: Install dependencies
       run: |
         python -m pip install --upgrade pip setuptools
-        python -m pip install coverage pytest
-        python -m pip install .
+        python -m pip install .[dev]
     - name: Tests
       run: |
         coverage run --branch -m pytest