CI: mypy: add mypy test to github actions

This adds a very basic CI run for type checking
on ubuntu-latest in the github actions pipelines.

Author: tirthasheshpatel

.github/workflows/linux.yml

@@ -79,3 +79,33 @@ jobs:
     - name: Test SciPy
       run: |
         python3.9 -u runtests.py -m fast
+
+  test_mypy:
+    name: Type Checking
+    if: "github.repository == 'scipy/scipy' && !contains(github.event.head_commit.message, '[ci skip]') && !contains(github.event.head_commit.message, '[skip ci]') && !contains(github.event.head_commit.message, '[skip github]') && !contains(github.ref, 'maintenance/')"
+    runs-on: ubuntu-latest
+    strategy:
+      matrix:
+        python-version: [3.9]
+
+    steps:
+    - uses: actions/checkout@v2
+
+    - name: Install Python 3.9
+      if: matrix.python-version == '3.9'
+      run: |
+        sudo apt-get update
+        sudo apt-get install -y --no-install-recommends python3.9-dev python3.9-distutils python3.9-venv
+        python3.9 -m pip install --upgrade pip setuptools
+
+    - name: Install other build dependencies
+      run: |
+        sudo apt-get install libatlas-base-dev liblapack-dev gfortran libgmp-dev libmpfr-dev libsuitesparse-dev ccache libmpc-dev
+
+    - name: Install packages
+      run: |
+        python3.9 -m pip install --user numpy setuptools wheel cython pytest pytest-xdist pybind11 pytest-xdist pythran
+
+    - name: Mypy
+      run: |
+        python3.9 -u runtests.py --mypy