Split Python linting into separate jobs.

This is just the first step to prove the concept. Most of the work done
in our workflows can be split into separate jobs to parallelize the
workflow. This will also make the checks page more readable.

This change alone probably won't speed up CI much.

.github/workflows/lint.yml

@@ -0,0 +1,28 @@
+name: Python lint
+
+on: workflow_call
+
+jobs:
+  black:
+    name: Black
+    runs-on: ubuntu-latest
+    steps:
+      - uses: actions/checkout@v2
+      - uses: actions/setup-python@v2
+      - uses: psf/black@stable
+        with:
+          args: ". --check"
+
+  mypy:
+    name: Type checking
+    runs-on: windows-latest
+    steps:
+      - uses: actions/checkout@v2
+        with:
+          submodules: true
+
+      - name: Set up Python environment
+        uses: ./.github/actions/setup-liberation-python
+
+      - name: mypy
+        uses: ./.github/actions/mypy