Test on Python 3.10 (#212)

Author: jasonpaulos

.github/workflows/build.yml

@@ -12,7 +12,7 @@ jobs:
     container: python:${{ matrix.python }}-slim
     strategy:
       matrix:
-        python: ['3.6', '3.7', '3.8', '3.9']
+        python: ['3.6', '3.7', '3.8', '3.9', '3.10']
     steps:
       - run: python3 --version
       - name: Check out code
@@ -53,6 +53,7 @@ jobs:
           path: docs/pyteal.docset.tar.gz
   upload-to-pypi:
     runs-on: ubuntu-20.04
+    container: python:3.9
     needs: ['build-test']
     if: ${{ github.event_name == 'push' && startsWith(github.ref, 'refs/tags') }}
     steps: