Summary

RequestsCookieJar currently inherits from both CookieJar and
MutableMapping, but it does not actually satisfy the MutableMapping
contract.

CookieJar.__iter__ yields Cookie objects, while MutableMapping expects
iteration over keys. That means RequestsCookieJar can report itself as a
MutableMapping even though code written against that interface will fail.

This change removes the MutableMapping base class from
RequestsCookieJar while keeping its existing dict-like convenience methods.

What changed

• removed MutableMapping from RequestsCookieJar's base classes
• kept the existing dict-like API (get, set, keys, items, etc.)
• added explicit implementations for behavior that had been inherited from
  MutableMapping:
  • __contains__
  • update
  • __eq__
• added a regression test to verify RequestsCookieJar is no longer treated
  as a MutableMapping

Why this approach

I considered changing iteration to yield keys instead, but that would also
change the current behavior of iterating over a cookie jar, which today yields
Cookie objects.

Removing the incorrect ABC inheritance seemed safer than changing iteration
semantics.

Testing

Ran a focused local test slice covering the changed behavior:

• tests/test_requests.py -k 'prepared_copy or cookie_jar_is_not_a_mutable_mapping or cookie_as_dict or cookie_parameters or cookie_duplicate_names'

Result:

• 13 passed

I also compared a full local test run on a clean clone versus this branch.

Clean clone:

• 400 passed
• 15 skipped
• 1 xfailed
• 196 errors

Patched branch:

• 401 passed
• 15 skipped
• 1 xfailed
• 196 errors

The extra passing test is the new regression test. The 196 errors were already
present on a clean clone in my environment.

Closes #7228