fix(maths): defer Matrix type annotations so import does not NameError

Importing maths.matrix_exponentiation raised 'NameError: name Matrix is
not defined' because __mul__ and modular_exponentiation annotate with
Matrix before the class name is bound (annotations evaluate eagerly at
class-body execution time).

Add 'from __future__ import annotations' to defer annotation evaluation,
matching the pattern used elsewhere in the repo.

Verified on current master: import succeeds (NameError gone) and all 10
doctests pass.

Signed-off-by: sudo-ai-git <sudo-ai-git@users.noreply.github.com>

Author: sudo-ai-git

maths/matrix_exponentiation.py

@@ -5,6 +5,8 @@
 https://www.hackerearth.com/practice/notes/matrix-exponentiation-1/
 """
 
+from __future__ import annotations
+
 import timeit