make mypy its own workflow

.github/workflows/mypy.yml

@@ -0,0 +1,14 @@
+name: Mypy
+on:
+  pull_request:
+  push:
+    branches: ["main"]
+jobs:
+  check-code-style:
+    runs-on: ubuntu-latest
+    steps:
+      - uses: actions/checkout@v2
+      - uses: actions/setup-python@v2
+        with:
+          python-version: '3.6'
+      - run: mypy

.github/workflows/pipeline.yml

@@ -15,7 +15,6 @@ jobs:
       - run: black --check --diff sydent/ tests/ matrix_is_test/ scripts/ setup.py
       - run: flake8 sydent/ tests/ matrix_is_test/ scripts/ setup.py
       - run: isort --check-only --diff sydent/ tests/ matrix_is_test/ scripts/ setup.py
-      - run: mypy
 
   run-unit-tests:
     needs: [check-code-style]