fix mypy type error in meta.py (#8509)

### Problem

`pants.util.meta.frozen_after_init` has a couple mypy type checking errors, specifically at: https://github.com/pantsbuild/pants/blob/00c4f8cc4a997f80a34f78b6d5fd6fca746befa7/src/python/pants/util/meta.py#L140-L144

This caused a failure on master, see https://travis-ci.org/pantsbuild/pants/jobs/600466692.

### Solution

- Add `# type: ignore` to the offending lines until #8496 is merged.

### Result

CI is unbroken!

src/python/pants/util/meta.py

@@ -137,8 +137,8 @@ def new_setattr(self, key: str, value: Any) -> None:
 
   cls.__init__ = new_init  # type: ignore
   cls.__setattr__ = new_setattr  # type: ignore
-  setattr(cls, frozen_after_init.sentinel_attr, True)
+  setattr(cls, frozen_after_init.sentinel_attr, True) # type: ignore
 
   return cls
 
-frozen_after_init.sentinel_attr = '_frozen_after_init'
+frozen_after_init.sentinel_attr = '_frozen_after_init' # type: ignore

tests/python/pants_test/engine/test_scheduler.py

@@ -428,5 +428,5 @@ def f(x: NonFrozenDataclass) -> int:
     with self.assertRaisesWithMessage(TypeError, dedent("""\
       @rule output type <class 'pants_test.engine.test_scheduler.NonFrozenDataclass'> is a dataclass declared without `frozen=True`, or without both `unsafe_hash=True` and the `@frozen_after_init` decorator! The engine requires that fields in params are immutable for stable hashing!""")):
       @rule
-      def f(x: int) -> NonFrozenDataclass:
+      def g(x: int) -> NonFrozenDataclass:
         return NonFrozenDataclass(x=x)