Explicitly export status categories

[medihack]

Describe the bug
The status categories are currently defined in pynetdicom._globals. Unfortunately, pyright complains when importing from a private module. The categories are also imported in pynetdicom.status, but not explicitly exported (__all__). So, pyright does complain when importing from there, too.

Expected behavior
Make the status categories importable conform to type checkers.

Steps To Reproduce
Have from pynetdicom._globals import STATUS_SUCCESS or from pynetdicom.status import STATUS_SUCCESS and run pyright type checker.

How to solve
Add to pynetdicom.status.

__all__ = [
    "STATUS_CANCEL",
    "STATUS_FAILURE",
    "STATUS_PENDING",
    "STATUS_SUCCESS",
    "STATUS_UNKNOWN",
    "STATUS_WARNING",
]

I can provide a PR if this change is acceptable.

[scaramallion]

Yeah, I should probably avoid using the single underscore modules (mostly looks tidier to me that way).