Add types in manager.py

Author: davidbrochart

.github/workflows/main.yml

@@ -31,6 +31,8 @@ jobs:
         pip install --upgrade setuptools pip
         pip install --upgrade --upgrade-strategy eager --pre -e .[test] pytest-cov codecov 'coverage<5'
         pip freeze
+    - name: Check types
+      run: mypy jupyter_client/manager.py
     - name: Run the tests
       run: py.test --cov jupyter_client -v jupyter_client
     - name: Code coverage