chore: use gapic-generator-python 0.50.4

Google APIs · copybara-github · commit dd1ef2849564 · 2021-07-09T14:58:31.000-07:00
PiperOrigin-RevId: 383929493
diff --git a/WORKSPACE b/WORKSPACE
@@ -224,7 +224,7 @@ load("@rules_python//python:pip.bzl", "pip_repositories")
 
 pip_repositories()
 
-_gapic_generator_python_version = "0.50.3"
+_gapic_generator_python_version = "0.50.4"
 
 http_archive(
     name = "gapic_generator_python",
