pypi: Include py.typed in MANIFEST.in.

According to the `setuptools` docs, once `include_package_data=True`
is passed to `setup()`, it will only include package data specified
in `MANIFEST.in`, and will ignore the `package_data` argument passed
to `setup()`. Therefore, `py.typed` was not included in our latest
PyPI release 0.8.1.

Since we specify all of our package data in the `MANIFEST.in` file
already, it makes more sense to include mention `py.typed` there so
that it is included in the expected fashion.

See pypa/setuptools#1461 for background.

Fixes #732.

zulip/MANIFEST.in

@@ -1,3 +1,4 @@
 recursive-include integrations *
 include README.md
 include zulip/examples/*
+include zulip/py.typed

zulip/setup.py

@@ -63,7 +63,6 @@ def recur_expand(target_root: Any, dir: Any) -> Generator[Tuple[str, List[str]],
             "zulip-api=zulip.cli:cli",
         ],
     },
-    package_data={"zulip": ["py.typed"]},
 )  # type: Dict[str, Any]
 
 setuptools_info = dict(