Add Github Action to run mypy / reorder-python-imports / black on all PRs (#251)

Also fixes import ordering (previously, local imports weren't grouped together as they should have been)

.github/workflows/pr-python-checks.yml

@@ -0,0 +1,41 @@
+name: Python Checks
+
+on:
+  pull_request:
+    branches: [ main ]
+
+jobs:
+  mypy-check:
+    runs-on: ubuntu-latest
+
+    steps:
+    - name: Checkout code
+      uses: actions/checkout@v3
+    
+    - name: Set up Python
+      uses: actions/setup-python@v4
+      with:
+        python-version: '3.11'
+        cache: 'pip'
+        cache-dependency-path: |
+          backend/requirements/default.txt
+          backend/requirements/dev.txt
+    - run: | 
+        python -m pip install --upgrade pip
+        pip install -r backend/requirements/default.txt
+        pip install -r backend/requirements/dev.txt
+
+    - name: Run MyPy
+      run: |
+        cd backend
+        mypy .
+
+    - name: Check import order with reorder-python-imports
+      run: |
+        cd backend
+        find ./danswer -name "*.py" | xargs reorder-python-imports --py311-plus
+
+    - name: Check code formatting with Black
+      run: |
+        cd backend
+        black --check .