Add InterpreterLocal type as escape hatch from freeze system

[mjp41]

InterpreterLocal provides per-sub-interpreter mutable state behind an
immutable indirection. When an object graph is frozen, InterpreterLocal
appears immutable to the freezer via tp_reachable (which visits only the
type and frozen default/factory fields), while each sub-interpreter
independently reads and writes its own value through get()/set() methods.

Two construction forms:
InterpreterLocal(42) - immutable default (frozen at construction)
InterpreterLocal(lambda: []) - factory callable (frozen at construction)

The type is a heap type created via PyType_FromModuleAndSpec, with
per-interpreter storage in the _immutable module's per-interpreter state.
This avoids changes to core interpreter structures.

[xFrednet]

LGTM, Shall we merge it?

[xFrednet]

The CI error looks suspicious: Could you take a look at it before merging?

8
Re-running test_freeze in verbose mode (matching: test_object_without_dict_uses_ob_flags)
Warning -- Unraisable exception
Exception ignored while finalizing file <_io.FileIO name='/home/runner/work/cpython/cpython-ro-srcdir/Lib/test/test_freeze/test_io.py' mode='rb' closefd=True>:
Traceback (most recent call last):
  File "/home/runner/work/cpython/cpython-ro-srcdir/Lib/test/libregrtest/single.py", line 49, in _filter_suite
    _filter_suite(test, pred)
    ~~~~~~~~~~~~~^^^^^^^^^^^^
ResourceWarning: unclosed file <_io.BufferedReader name='/home/runner/work/cpython/cpython-ro-srcdir/Lib/test/test_freeze/test_io.py'>
Warning -- Unraisable exception
Exception ignored while finalizing file <_io.FileIO name='/home/runner/work/cpython/cpython-ro-srcdir/Lib/test/test_freeze/test_io.py' mode='rb' closefd=True>:
Traceback (most recent call last):
  File "/home/runner/work/cpython/cpython-ro-srcdir/Lib/test/libregrtest/single.py", line 49, in _filter_suite
    _filter_suite(test, pred)
    ~~~~~~~~~~~~~^^^^^^^^^^^^
ResourceWarning: unclosed file <_io.TextIOWrapper name='/home/runner/work/cpython/cpython-ro-srcdir/Lib/test/test_freeze/test_io.py' mode='r' encoding='utf-8'>
test_object_without_dict_uses_ob_flags (test.test_freeze.test_set_freezable.TestSetFreezableEdgeCases.test_object_without_dict_uses_ob_flags) ... ok

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Ran 1 test in 0.001s