Run lint on all Python versions

.github/workflows/continuous-integration.yml

@@ -67,6 +67,12 @@ jobs:
           fail_ci_if_error: false 
   lint:
     runs-on: ubuntu-latest
+    strategy:
+      # Allow other matrix jobs to complete if 1 fails
+      fail-fast: false
+      matrix:
+        python-version: ["3.6", "3.7", "3.8"]
+
     steps:
       - uses: actions/checkout@v2
 
@@ -75,7 +81,7 @@ jobs:
         with:
           python-version: ${{ matrix.python-version }}
 
-      - name: Cache dependencies (${{ runner.os }})
+      - name: Cache dependencies
         uses: actions/cache@v2
         with:
           path: ~/.cache/pip
@@ -85,7 +91,7 @@ jobs:
       - name: Install dependencies
         run: |
           pip install -r requirements-dev.txt
-          pip install -e ".[validation]"
+          pip install -e .
 
       - name: Execute linters & type checkers
         run: ./scripts/lint