Fix python release breakages by set MB_PYTHON_OSX_VER (protocolbuffers#7038)

Author: anandolee

kokoro/release/python/macos/build_artifacts.sh

@@ -50,8 +50,13 @@ build_artifact_version() {
   mv wheelhouse/* $ARTIFACT_DIR
 }
 
+export MB_PYTHON_OSX_VER=10.9
 build_artifact_version 2.7
-build_artifact_version 3.5
 build_artifact_version 3.6
 build_artifact_version 3.7
 build_artifact_version 3.8
+
+# python OSX10.9 does not have python 3.5
+export MB_PYTHON_OSX_VER=10.6
+build_artifact_version 3.5
+