Fix undefined name "Future" --> asyncio.Future

[cclauss]

% ruff check --output-format=github --select=E9,F63,F7,F82 --target-version=py39

Error: sdk/python/packages/flet-core/src/flet_core/page.py:926:11: F821 Undefined name `Future`

from __future__ import annotations  # Only needed for Python 3.8 which is now EOL.

Description

Fixes #(issue)

Test Code

# Test code for the review of this PR

Type of change

• [ ] Bug fix (non-breaking change which fixes an issue)
• [ ] New feature (non-breaking change which adds functionality)
• [ ] Breaking change (fix or feature that would cause existing functionality to not work as expected)
• [ ] This change requires a documentation update

Checklist:

• [x] I signed the CLA.
• [x] My code follows the style guidelines of this project
• [x] I have performed a self-review of my own code
• [ ] I have commented my code, particularly in hard-to-understand areas
• [ ] My changes generate no new warnings

• [ ] New and existing tests pass locally with my changes
• [ ] I have made corresponding changes to the documentation (if applicable)

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