Summary:
Improve equality assertion explanations for objects implementing collections.abc.Mapping, not only concrete dict instances.

Previously, non-dict mapping objects could fall through to the generic iterable comparison path, even though the existing mapping comparison logic already works with the Mapping protocol. This updates the dispatch check accordingly and renames the internal helper from dict-specific wording to mapping-specific wording.

Closes #14461.

Details:
This keeps the change intentionally narrow:

• use Mapping for the assertion comparison dispatch
• rename the internal helper to match the broader behavior
• add a focused regression test with a small custom Mapping
• add a changelog fragment
• add myself to AUTHORS

I avoided adding coverage for third-party mapping libraries here, since those can have different key/value semantics and would make this PR broader than necessary.

Tests:

PYTHONPATH=src .venv/bin/python -m pytest \
  testing/test_assertion.py::TestAssert_reprcompare::test_dict \
  testing/test_assertion.py::TestAssert_reprcompare::test_dict_omitting \
  testing/test_assertion.py::TestAssert_reprcompare::test_dict_omitting_with_verbosity_1 \
  testing/test_assertion.py::TestAssert_reprcompare::test_dict_omitting_with_verbosity_2 \
  testing/test_assertion.py::TestAssert_reprcompare::test_dict_different_items \
  testing/test_assertion.py::TestAssert_reprcompare::test_mapping_different_items \
  -q -s

Result: 6 passed

PYTHONPATH=src .venv/bin/python -m pytest \
  testing/test_assertion.py::TestAssert_reprcompare \
  -k "dict or mapping" \
  -q -s

Result: 8 passed, 34 deselected