Switch frozen-instance error type from TypeError to dataclasses.FrozenInstanceError

[Technologicat]

Spun off from #35 (closed in 2.2.0): the immutability cleanup for unpythonic.llist.cons (and any future "frozen" types in this codebase) shipped raising TypeError to preserve API stability under the 2.x compatibility promise.

Standard library convention since Python 3.7 is dataclasses.FrozenInstanceError for "tried to mutate a frozen instance." Aligning is desirable for ecosystem consistency, but FrozenInstanceError subclasses AttributeError, not TypeError. Switching is therefore a breaking change for any user code catching TypeError from a cons mutation attempt — appropriate for a major-version bump.

Scope

• unpythonic/llist.py: cons.__setattr__ and cons.__delattr__ raise dataclasses.FrozenInstanceError instead of TypeError.
• Tests: update unpythonic/tests/test_llist.py immutability assertions to expect FrozenInstanceError (or AttributeError).
• CHANGELOG: Changed (breaking) entry under 3.0.0.
• Audit any other "frozen" types added between now and 3.0.0 for the same alignment.

Notes

• assignonce.__delattr__ (added in 2.2.0) raises AttributeError, which already aligns naturally — leave alone.
• frozendict does not currently lock down attribute writes (only mapping writes); this issue is about types whose contract is "frozen instance."

— Claude (Opus 4.7)

[Technologicat]

Update: shim approach landed in 2.2.0. unpythonic.llist.FrozenAttributeError multiply-inherits from TypeError and dataclasses.FrozenInstanceError; cons raises this. Either except TypeError or except FrozenInstanceError catches.

Scope of this issue therefore shifts:

• 3.0.0 work is now to drop the TypeError base from FrozenAttributeError (or replace usages with plain FrozenInstanceError and remove the shim class).
• The compat-during-2.x problem is solved; this is a clean stdlib-alignment step for 3.0.0.

— Claude (Opus 4.7)