run mypy on github action

Travis is getting too slow to start, so migrating away some small steps to regain some CI time.

.github/workflows/mypy.yml

@@ -0,0 +1,30 b''
+name: Run MyPy
+
+on:
+  push:
+    branches: [ master ]
+  pull_request:
+    branches: [ master ]
+
+jobs:
+  build:
+
+    runs-on: ubuntu-latest
+    strategy:
+      matrix:
+        python-version: [3.8]
+
+    steps:
+    - uses: actions/checkout@v2
+    - name: Set up Python ${{ matrix.python-version }}
+      uses: actions/setup-python@v2
+      with:
+        python-version: ${{ matrix.python-version }}
+    - name: Install dependencies
+      run: |
+        python -m pip install --upgrade pip
+        pip install mypy
+    - name: Lint with mypy
+      run: |
+        mypy IPython/terminal/ptutils.py
+        mypy IPython/core/c*.py