Fix matrix exponentiation type annotations - #15076

Describe your change:

Fixes a NameError in Python/maths/matrix_exponentiation.py caused by self-referential type annotations in the Matrix class.

The Matrix.__mul__ method references Matrix in its parameter and return type annotations before the class definition has been fully evaluated. This causes Python to raise NameError: name 'Matrix' is not defined when the module is imported.

This change adds from __future__ import annotations to postpone the evaluation of type annotations, allowing the self-referential Matrix annotations to be resolved correctly without changing the algorithm's behavior.

• Add an algorithm?
• Fix a bug or typo in an existing algorithm?
• Add or change doctests?
• Documentation change?

Fixes #15075

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".