fix: NameError when importing matrix_exponentiation.py

[Georgefifth]

Describe your change:

maths/matrix_exponentiation.py raises NameError: name 'Matrix' is not defined on import: Matrix.__mul__ uses Matrix in its own annotation, which is evaluated while the class body is still executing (introduced by the type hints in #14288). The fix adds from __future__ import annotations to defer annotation evaluation, matching the convention already used in 250+ files in this repo (e.g. maths/prime_factors.py). Two-line diff, no behavior change.

Verified: before the fix the import fails with the reported NameError; after the fix the import succeeds and python3 -m doctest maths/matrix_exponentiation.py passes (10/10). I also scanned all of maths/ for the same pattern and this is the only occurrence.

• Add an algorithm?
• Fix a bug or typo in an existing algorithm?
• Add or change doctests? -- Note: Please avoid changing both code and tests in a single pull request.
• Documentation change?

Checklist:

• I have read CONTRIBUTING.md.
• This pull request is all my own work -- I have not plagiarized.
• I know that pull requests will not be merged if they fail the automated tests.
• This PR only changes one algorithm file. To ease review, please open separate PRs for separate algorithms.
• All new Python files are placed inside an existing directory.
• All filenames are in all lowercase characters with no spaces or dashes.
• All functions and variable names follow Python naming conventions.
• All function parameters and return values are annotated with Python type hints.
• All functions have doctests that pass the automated testing.
• All new algorithms include at least one URL that points to Wikipedia or another similar explanation.
• If this pull request resolves one or more open issues then the description above includes the issue number(s) with a closing keyword: "Fixes #ISSUE-NUMBER".

Fixes #15075