fix: NameError when importing matrix_exponentiation.py

[Georgefifth]

Problem

Importing maths/matrix_exponentiation.py raises NameError: name 'Matrix' is not defined. Matrix.__mul__ uses Matrix in its own annotation, which is evaluated while the class body is still executing, before the name exists. The type hints were added in #14288.

Solution

Add from __future__ import annotations to defer annotation evaluation. This is the same fix already used in 250+ files in this repo (e.g. maths/prime_factors.py). Two-line diff, no behavior change.

Verification

• Before: python3 -c 'import maths.matrix_exponentiation' fails with the reported NameError.
• After: import succeeds and python3 -m doctest maths/matrix_exponentiation.py passes (10/10).
• ruff check passes.

Also scanned all of maths/ for the same pattern (self-referential annotations without the future import); this is the only occurrence.

Closes #15075

[algorithms-keeper]

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