Add mypy to CI.

.github/workflows/ci.yaml

@@ -3,6 +3,15 @@ name: Test tesla_api
 on: pull_request
 
 jobs:
+  mypy:
+    name: Check annotations with Mypy
+    runs-on: ubuntu-latest
+    steps:
+      - uses: actions/checkout@v2
+      - uses: actions/setup-python@v2
+      - run: pip install aiohttp mypy
+      - run: mypy
+
   test:
     name: Tests
     runs-on: ubuntu-latest