Declare more functions in importlib.metadata.PackageMetadata

Feature or enhancement

Add get(), keys(), and potentially more fields from Mapping to PackageMetadata. Also get_payload().

Pitch

It is common for PackageMetadata usages to incorporate calls like get() and keys(), which the protocol are currently missing.

There is also a style of metadata that uses the payload as the long description. Current tools tend to use get_payload() to retrieve the data (implemented by email.mail.Message), but the function is also missing from the protocol.

Previous discussion

N/A

importlib.metadata.PackageMetadata を見つけ、そのプロトコルを Mapping および email.mail.Message と比較します。get()、keys()、get_payload() がどのように表現されているかを確認し、その後、これらのインターフェースが利用可能であることを確認する、要求された宣言とテストを追加します。