Implement EntryPoint as a simple object (remove namedtuple)

[jaraco]

Fixes #337 by eschewing the namedtuple superclass. Allows for removal of PyPy workaround and incidental namedtuple interfaces.

[RonnyPfannschmidt]

I would recommend going for explicit assignment here for the sake of readability and setting self.dist as well

[jaraco]

I'd actually like to keep the name/group/value as the essential attributes separate from the dist, which is optional and auxiliary. Agree with explicit assignment. I'm not sure it's possible.

[jaraco]

I'm a little reluctant to move forward with this implementation primarily because of concerns about compatibility. This change removes the ability to construct EntryPoint objects from three positional args. I'm debating whether this change can be introduced without supporting that interface, or if the positional args is worth retaining, or if it should be deprecated.

I'm aiming for "preferably one obvious way" to initialize an EntryPoint to limit the support surface. I'm even tempted to disallow construction of EntryPoint objects altogether.

Maybe the best approach is to implement a simple constructor, retain compatibility, and consider constraining the interface later. Edit: This approach is the one this PR enacts, attempting to retain compatibility.

[mr-c]

Hello, this change broke stevedore which broke bandit for at least myself (but probably others)

https://github.com/common-workflow-language/cwltool/pull/1482/checks?check_run_id=3454232416#step:9:50
PyCQA/bandit#730

Reverting to importlib_metadata version 4.7.1 resolves the problem for me