Add type hint to read_text result.

importlib_metadata/__init__.py

@@ -31,7 +31,7 @@
 from importlib import import_module
 from importlib.abc import MetaPathFinder
 from itertools import starmap
-from typing import List, Mapping, Optional
+from typing import List, Mapping, Optional, cast
 
 
 __all__ = [
@@ -352,7 +352,7 @@ class Distribution(metaclass=abc.ABCMeta):
     """A Python distribution package."""
 
     @abc.abstractmethod
-    def read_text(self, filename):
+    def read_text(self, filename) -> Optional[str]:
         """Attempt to load metadata file given by the name.
 
         :param filename: The name of the file in the distribution info.
@@ -426,14 +426,15 @@ def metadata(self) -> _meta.PackageMetadata:
         The returned object will have keys that name the various bits of
         metadata.  See PEP 566 for details.
         """
-        text = (
+        opt_text = (
             self.read_text('METADATA')
             or self.read_text('PKG-INFO')
             # This last clause is here to support old egg-info files.  Its
             # effect is to just end up using the PathDistribution's self._path
             # (which points to the egg-info file) attribute unchanged.
             or self.read_text('')
         )
+        text = cast(str, opt_text)
         return _adapters.Message(email.message_from_string(text))
 
     @property