Support typing.assert_type.

* Aliases typing.assert_type to our assert_type pseudo-builtin.
* Removes single-argument assert_type form. This functionality didn't make it
  into the typing library variant. I don't think it's used much anyway,
  given that pytype_extensions.assert_type doesn't support it at runtime, and
  no one's ever noticed.

PiperOrigin-RevId: 577000283

Author: rchen152

pytype_extensions/__init__.py

@@ -180,7 +180,7 @@ def assert_type(__x, __t) -> None:  # pylint: disable=invalid-name
   Usage example:
 
   ```
-  import pytype_extensinos
+  import pytype_extensions
   assert_type = pytype_extensions.assert_type
 
   x = 3