Build mathlib in github actions

Author: reilleya

.github/workflows/tests.yml

@@ -20,6 +20,9 @@ jobs:
         python -m pip install --upgrade pip
         pip install flake8 pytest
         if [ -f requirements.txt ]; then pip install -r requirements.txt; fi
+    - name: Build mathlib
+      run: |
+        python setup.py build_ext --inplace
     - name: Lint with flake8
       run: |
         # stop the build if there are Python syntax errors or undefined names