remove skip of mypy on py36

Author: github-actions[bot]

.github/workflows/main.yml

@@ -112,7 +112,6 @@ jobs:
           pip freeze
 
       - name: Check types
-        if: ${{ matrix.python-version != '3.6' }}
         run: mypy jupyter_client --exclude '\/tests|kernelspecapp|ioloop|runapp' --install-types --non-interactive
 
       - name: Run the tests