Sync from Piper @480194141

PROTOBUF_SYNC_PIPER
diff --git a/python/BUILD.bazel b/python/BUILD.bazel
index ffaf988..56d8c8a 100644
--- a/python/BUILD.bazel
+++ b/python/BUILD.bazel
@@ -276,6 +276,13 @@
 )
 
 py_test(
+    name = "field_mask_test",
+    srcs = ["google/protobuf/internal/field_mask_test.py"],
+    imports = ["."],
+    deps = [":python_test_lib"],
+)
+
+py_test(
     name = "generator_test",
     srcs = ["google/protobuf/internal/generator_test.py"],
     imports = ["."],
@@ -453,12 +460,10 @@
         "google/protobuf/pyext/README",
         "google/protobuf/python_protobuf.h",
         "internal.bzl",
-        "mox.py",
         "python_version.py",
         "release.sh",
         "setup.cfg",
         "setup.py",
-        "stubout.py",
         "tox.ini",
     ],
     strip_prefix = strip_prefix.from_root(""),