Fix NameError: pp not defined at runtime in _operator_states.py

[Copilot]

import porepy as pp was guarded by TYPE_CHECKING, but pp.GridLikeSequence appeared in __init__ annotations of all three mixin classes (TimeDependentOperator, IterativeOperator, ReferenceOperator). Python evaluates these annotations at class definition time, causing NameError: name 'pp' is not defined on every import porepy — breaking all tests under Python 3.13.

Proposed changes

• Add from __future__ import annotations to _operator_states.py to defer annotation evaluation, keeping pp safely behind TYPE_CHECKING.

Types of changes

• Bugfix (non-breaking change which fixes an issue).

Checklist

• The documentation is up-to-date.
• Static typing is included in the update.
• This PR does not duplicate existing functionality.
• The update is covered by the test suite (including tests added in the PR).
• If new skipped tests have been introduced in this PR, pytest was run with the --run-skipped flag.

[keileg]

Included directly in #1696