Update build_cmake.yml

Fix python 3.8 for now rather than "latest"

Should perhaps consider testing multiple versions eventually? Don't need to re-run all the C++ tests from scratch for each python version though.
diff --git a/.github/workflows/build_cmake.yml b/.github/workflows/build_cmake.yml
index 12ab56a..c1470de 100644
--- a/.github/workflows/build_cmake.yml
+++ b/.github/workflows/build_cmake.yml
@@ -52,7 +52,7 @@
       - name: Set up Python 3.x
         uses: actions/setup-python@v2
         with:
-          python-version: '3.x' # grabs latest minor version of python3
+          python-version: '3.8' # 3.x grabs latest minor version of python3, but 3.9 not fully supported yet
       - name: Install Python dependencies
         run: python -m pip install --upgrade pip setuptools wheel numpy tox
       - name: Build and run Python tests