Add python3.8 in build_artifacts for linux and macos (#7019)

kokoro/release/python/linux/build_artifacts.sh

@@ -53,3 +53,4 @@ 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

kokoro/release/python/macos/build_artifacts.sh

@@ -54,3 +54,4 @@ 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