Use Python 3.10 beta 4 instead of beta 1 in CI (#5839)

Quote all Python versions to make sure they are interpreted as strings,
not floats.

.github/workflows/tests.yml

@@ -74,7 +74,7 @@ jobs:
     strategy:
       matrix:
         python-platform: ["Linux", "Windows", "Darwin"]
-        python-version: [3.6, 3.7, 3.8, 3.9, '3.10']
+        python-version: ["3.6", "3.7", "3.8", "3.9", "3.10"]
       fail-fast: false
     env:
       PYRIGHT_VERSION: 1.1.152  # Must match pyright_test.py.
@@ -100,7 +100,7 @@ jobs:
     strategy:
       matrix:
         os: ["ubuntu-latest", "windows-latest", "macos-latest"]
-        python-version: [3.6, 3.7, 3.8, 3.9, '3.10.0-beta.1']
+        python-version: ["3.6", "3.7", "3.8", "3.9", "3.10.0-beta.4"]
       fail-fast: false
 
     steps: