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

@@ -39,6 +39,7 @@
     import importlib_metadata
 
 metadata = importlib_metadata.metadata("twine")
+assert metadata is not None  # nosec: B101
 
 
 __title__ = metadata["name"]