Update build.yml

Author: torresc24

.github/workflows/build.yml

@@ -24,6 +24,7 @@ jobs:
         run: |
           python -m pip install --upgrade pip
           python -m pip install sphinx
+          pip install sphinx_rtd_theme
           pip install virtualenv==20.0.1
       - name: Install tox
         if: ${{ matrix.python-version }} != 3.4