fix: IntervalDict.pop(key, None) raises KeyError instead of returning None

[gaoflow]

What

IntervalDict.pop(key, default) uses default is None to detect whether a
default argument was supplied, so passing None explicitly is indistinguishable
from calling pop(key) with no default:

d = P.IntervalDict([(P.closed(0, 5), 'A')])

d.pop(10)        # KeyError — correct
d.pop(10, None)  # KeyError — wrong: should return None
d.pop(10, 'x')  # 'x'     — correct

Python's built-in dict.pop always returns the default when one is given,
regardless of its value.

Fix

Replace the None sentinel with a private _MISSING = object() sentinel so
that any explicitly supplied default — including None — is honoured.

Test

Added an assertion to test_pop_missing_value that pop(key, None) returns
None for a missing key rather than raising KeyError.

---

This pull request was prepared with the assistance of AI, under my direction and review.

[AlexandreDecan]

Thanks for proposing this "fix" (it wasn't an issue since the behaviour aligns with the documentation). Was there a human involved in the loop? Do you use this library? Your recent activity suggests (creating hundreds of PRs in dozens of unrelated projects) this was fully automatically generated by an AI.

[gaoflow]

Thanks for calling this out. You're right that I misread the documented contract here, so this PR should not be treated as a bug fix.

There was human review in the loop, but the patch was AI-assisted and I do not use this library in production. I should have been more conservative before opening the PR. Closing this to avoid taking more maintainer time.