fix: flask 2.X compatibility

[dpgaspar]

Currently there are no package constraints defined for Flask's version, yet flask-sqlalchemy-lite fails with:

  File "./venv/lib/python3.10/site-packages/flask_sqlalchemy_lite/_extension.py", line 12, in <module>
    from flask.sansio.app import App
ModuleNotFoundError: No module named 'flask.sansio'

Solution:
Since App is only used for type checking, this PR introduces a try/except import to fall back to Flask from flask.app in environments where flask.sansio.app is unavailable. This ensures retro compatibility without requiring users to upgrade Flask unnecessarily.

Impact:
This is a minimal and safe change that restores compatibility with Flask versions <3.0 while preserving type checking behavior for 3.x+.

[davidism]

This wasn't my intention when writing this extension, I missed adding a lower bound on Flask >= 3.1. Flask < 3.1 is no longer supported: https://palletsprojects.com/versions