[wpimath] Use jinja for codegen (#3574)

While not really needed for wpimath, it will make more complex codegen
in the future significantly easier.

.github/workflows/cmake.yml

@@ -10,7 +10,7 @@ jobs:
         include:
           - os: ubuntu-latest
             name: Linux
-            container: wpilib/roborio-cross-ubuntu:2021-18.04
+            container: wpilib/roborio-cross-ubuntu:2021-20.04
             flags: ""
           - os: macos-latest
             name: macOS
@@ -26,6 +26,12 @@ jobs:
           if [ "$RUNNER_OS" == "macOS" ]; then
             brew install opencv
           fi
+      - name: Set up Python 3.8
+        uses: actions/setup-python@v2
+        with:
+          python-version: 3.8
+      - name: Install jinja
+        run: python -m pip install jinja2
       - name: configure
         run: mkdir build && cd build && cmake ${{ matrix.flags }} ..
       - name: build