Fixes msgpack#723.

…() (msgpack#727)

Finding msgpack#6 of msgpack#683.

…on (msgpack#716)

…threaded builds (msgpack#729)

The Windows Python 3.15t Actions job was failing in the pure-Python
fallback test phase under `pytest-run-parallel`. The failure came from
`test_nest_limit_1024`, which mutates the process-wide recursion limit
and can race when multiple iterations run concurrently.

…sgpack#728)

Co-authored-by: Dylan Pulver <dylanpulver@users.noreply.github.com>

Support packing a Python 3.15 `frozendict`.

This PR doesn't support _unpacking_ into a `frozendict` other than with
the generic `object_pairs_hook=frozendict`.

---------

Signed-off-by: Edgar Ramírez Mondragón <edgarrm358@gmail.com>
Co-authored-by: Copilot Autofix powered by AI <175728472+Copilot@users.noreply.github.com>