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.