[pytype] Disable mypy type checking on third_party/markeupsafe target

Change-Id: Ie56dc96d387784a98e3d30f97fb2ae5dd79b0fcb
Reviewed-on: https://fuchsia-review.googlesource.com/c/third_party/markupsafe/+/1116439
Reviewed-by: Danny Rosen <dannyrosen@google.com>
Reviewed-by: Kevin Cho <chok@google.com>

BUILD.gn

diff --git a/BUILD.gn b/BUILD.gn
index b7fbec8..5eaea4d 100644
--- a/BUILD.gn
+++ b/BUILD.gn
@@ -5,6 +5,7 @@
 import("//build/python/python_library.gni")
 
 python_library("markupsafe") {
+  enable_mypy = false
   sources = [
     "__init__.py",
     "_compat.py",