Fix py3.9 mypy error in __init__ around PackageMetadata

On Python 3.9 ``importlib_metadata`` returns Optional[PackageMetadata]
instead of PackageMetadata, this leads to mypy complaining that
Optional[...] is not indexable and does not have ``get_all``. Instead,
let's just cast it for ourselves.

Author: sigmavirus24

twine/__init__.py

@@ -32,13 +32,16 @@
 
 import email.utils
 import sys
+import typing as t
 
 if sys.version_info >= (3, 10):
     import importlib.metadata as importlib_metadata
 else:
     import importlib_metadata
 
-metadata = importlib_metadata.metadata("twine")
+metadata = t.cast(  # type: ignore[redundant-cast]
+    importlib_metadata.PackageMetadata, importlib_metadata.metadata("twine")
+)
 
 
 __title__ = metadata["name"]