Make RequestsCookieJar.popitem() work

[vineethsaivs]

Closes #6190

RequestsCookieJar subclasses both CookieJar and MutableMapping[str, str | None]. Its __iter__ is inherited from CookieJar and yields Cookie objects rather than names. The popitem() it inherited from MutableMapping does roughly:

key = next(iter(self))   # a Cookie object, not a name
value = self[key]        # __getitem__ looks up by name -> KeyError

so popitem() always raised KeyError, even on a non-empty jar:

>>> jar = requests.cookies.RequestsCookieJar()
>>> jar.set("a", "1"); jar.set("b", "2")
>>> jar.popitem()
KeyError: "name=Cookie(version=0, name='a', ...), domain=None, path=None"

This overrides popitem() to remove and return a (name, value) pair using the jar's existing dict-like view (iteritems()), and to raise KeyError only when the jar is empty, matching the documented behavior and the MutableMapping[str, str | None] contract.

Added a regression test (test_cookie_popitem) that pops both entries, checks each returned pair is a real (name, value) from the jar, and confirms a KeyError on the empty jar. It fails on main and passes with this change.

[avinashkamat48]

popitem() removes by cookie name only (del self[name]) after reading the first (name, value) pair. That can remove more than the single cookie returned when the jar contains multiple cookies with the same name on different domains/paths, or hit the existing duplicate-name conflict behavior. Please add a test with two cookies sharing a name but differing by domain/path and remove the exact cookie object selected by the iterator rather than deleting by name alone.

[vineethsaivs]

Good catch, fixed. popitem() now selects the cookie via the iterator and removes that exact object with clear(cookie.domain, cookie.path, cookie.name), so it can no longer drop other cookies that happen to share the name on a different domain or path. Added a regression test (test_cookie_popitem_removes_only_the_selected_cookie) that puts two cookies named dup on different domains and asserts popitem() removes exactly the one it returns and leaves the other intact.