fix python windows artifact build

tools/run_tests/artifacts/build_artifact_python.bat

@@ -16,7 +16,8 @@
 set PATH=C:\%1;C:\%1\scripts;C:\msys64\mingw%2\bin;C:\tools\msys64\mingw%2\bin;%PATH%
 
 pip install --upgrade six
-pip install --upgrade setuptools
+@rem some artifacts are broken for setuptools 38.5.0. See https://github.com/grpc/grpc/issues/14317
+pip install --upgrade setuptools==38.2.4
 pip install -rrequirements.txt
 
 set GRPC_PYTHON_BUILD_WITH_CYTHON=1