fix: add back support for Python 3.9

[yuqi1129]

This pull request introduces two main changes: it updates the test matrix in the GitHub Actions workflow to include Python 3.9, and it adds the from __future__ import annotations import to several filter modules. These updates improve compatibility with older Python versions and prepare the codebase for future Python enhancements.

[Copilot]

Pull request overview

Restores Python 3.9 compatibility by ensuring CI runs against 3.9 and by making filter modules safe to import on 3.9 when using PEP 604-style type hints (e.g., str | bytes).

Changes:

• Add Python 3.9 to the GitHub Actions test matrix for Linux/macOS/Windows.
• Add from __future__ import annotations to filter modules that use X | Y type annotations to prevent runtime evaluation errors on Python 3.9.

Reviewed changes

Copilot reviewed 5 out of 5 changed files in this pull request and generated no comments.

Show a summary per file

File	Description
src/banks/filters/video.py	Postpone annotation evaluation to keep `str
src/banks/filters/image.py	Same: adds future import to support PEP 604 union hints on 3.9.
src/banks/filters/document.py	Same: adds future import to avoid runtime type-evaluation issues on 3.9.
src/banks/filters/audio.py	Same: adds future import to support PEP 604 union hints on 3.9.
.github/workflows/test.yml	Expands CI matrix to include Python 3.9 across all OS jobs.

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[coveralls]

coverage: 94.763% (+0.02%) from 94.742%
when pulling 0930452 on yuqi1129:issue_70
into 05ff553 on masci:main.