Tighten the EntryPoint interface

I've also observed that the issue affects .module and .attr and .load. I'm trying to decide what's the best behavior here. I think it should fail early, at construction time, but right now, the interface is open to mutation of the entry point properties. Probably those should be made final.

Originally posted by @jaraco in #488

Additionally, EntryPoint could probably subclass SimpleNamespace except that EntryPoint currently allows for positional arguments. Let's deprecate that behavior and require keyword arguments.

EntryPoint の定義から始め、現在の構築およびミューテーションの挙動を調査します。提案された final properties、keyword-only construction、positional-argument deprecation を受け入れ基準として使用します。インターフェースの挙動と deprecation policy が確定し、検証されれば完了です。