Clean up GitHub Actions workflow (#10)

* Manage import order using isort

* Install necessary packages

* Clean up the GitHub Actions workflow

* Add a pypy build

* Fix stray colon

Author: mdickinson

.github/workflows/on-commit.yml

@@ -0,0 +1,48 @@
+# Run tests and style checks on each commit to an open pull request.
+
+name: on-commit
+
+on: pull_request
+
+jobs:
+  test:
+    runs-on: ubuntu-latest
+
+    strategy:
+      matrix:
+        python-version: ['3.6', '3.9', '3.10-dev', 'pypy-3.7']
+
+    steps:
+    - name: Check out the code
+      uses: actions/checkout@v2
+    - name: Set up Python ${{ matrix.python-version }}
+      uses: actions/setup-python@v2
+      with:
+        python-version: ${{ matrix.python-version }}
+    - name: Test with unittest
+      run: |
+        python -m unittest
+
+  style:
+    runs-on: ubuntu-latest
+
+    strategy:
+      matrix:
+        python-version: ['3.9']
+
+    steps:
+    - name: Check out the code
+      uses: actions/checkout@v2
+    - name: Set up Python ${{ matrix.python-version }}
+      uses: actions/setup-python@v2
+      with:
+        python-version: ${{ matrix.python-version }}
+    - name: Install dependencies
+      run: |
+        python -m pip install --upgrade pip
+        python -m pip install black flake8 isort
+    - name: Run style checks
+      run: |
+        python -m flake8 .
+        python -m isort --check .
+        python -m black --check .