Fix pickling of NewType instances defined in __main__ on Python 3.10+

[charan-rathore]

Fixes #520.

On Python 3.10 and newer, typing.NewType is a class and its instances define
__reduce__ returning their qualified name. cloudpickle therefore pickles them
by reference. When the NewType is defined in __main__ (a script or a
notebook), loading the pickle in a fresh process fails with
AttributeError: Can't get attribute 'RunDict' on <module '__main__' ...>.

This PR registers a dedicated reducer for typing.NewType, following the
_typevar_reduce shape @ogrisel suggested in the issue:

• NewType instances that are importable from their module are still pickled by
  reference.
• Dynamic ones (defined in __main__, or in modules registered with
  register_pickle_by_value) are reconstructed by value with _make_newtype
  and recorded in the dynamic class tracker, so identity is preserved across
  processes and within a single pickle stream.
• On Python older than 3.10, typing.NewType is a function and the existing
  function pickling path already works, so the reducer is only registered when
  typing.NewType is a class.

Name, qualname, module and supertype are all preserved, and nested supertypes
(a NewType whose supertype is another NewType) roundtrip correctly.

Tests:

• New regression tests cover: fresh-process load of a __main__ NewType (the
  exact cloudpickle fails to pickle NewType in main module by value on Python 3.10 and newer #520 repro), identity across a subprocess echo, memoization within one
  pickle stream, nested supertypes, importable NewType by reference, and
  modules registered with register_pickle_by_value.
• All new dynamic-NewType tests fail on current master (PicklingError /
  AttributeError) and pass with this change.
• Full suite on Python 3.10.12: 285 passed, 3 skipped, 2 xfailed.
• Focused typing tests pass on Python 3.12, 3.13 and 3.14.

Caveats: the 3.8/3.9 guard (isinstance(typing.NewType, type)) is untested locally because no such interpreter was available. black --check flags tests/cloudpickle_ipykernel_test.py on unpatched master too (existing formatting drift); this change does not touch that file.