Add Python3.8 to our macOS CI

tools/internal_ci/helper_scripts/prepare_build_macos_rc

@@ -58,6 +58,10 @@ export PYTHONPATH=/Library/Python/3.4/site-packages
 time curl -O https://www.python.org/ftp/python/3.7.0/python-3.7.0-macosx10.9.pkg
 time sudo installer -pkg ./python-3.7.0-macosx10.9.pkg -target /
 
+# Install Python 3.8
+time curl -O https://www.python.org/ftp/python/3.8.0/python-3.8.0-macosx10.9.pkg
+time sudo installer -pkg ./python-3.8.0-macosx10.9.pkg -target /
+
 # set xcode version for Obj-C tests
 sudo xcode-select -switch /Applications/Xcode_9.2.app/Contents/Developer/
 

tools/run_tests/artifacts/artifact_targets.py

@@ -406,8 +406,7 @@ def targets():
         PythonArtifact('macos', 'x64', 'python3.5'),
         PythonArtifact('macos', 'x64', 'python3.6'),
         PythonArtifact('macos', 'x64', 'python3.7'),
-        # TODO(https://github.com/grpc/grpc/issues/20615) Enable this artifact
-        # PythonArtifact('macos', 'x64', 'python3.8'),
+        PythonArtifact('macos', 'x64', 'python3.8'),
         PythonArtifact('windows', 'x86', 'Python27_32bits'),
         PythonArtifact('windows', 'x86', 'Python35_32bits'),
         PythonArtifact('windows', 'x86', 'Python36_32bits'),