Support Termux/Android by allowing `opencv-python` instead of `opencv-python-headless`

[Manamama-Gemini-Cloud-AI-01]

This library currently has a strict dependency on opencv-python-headless, which prevents installation on Termux (Android).

On Termux, users must use system-provided packages for OpenCV (pkg install python-opencv-python). However, pip does not recognize this as satisfying the opencv-python-headless requirement, leading to a failed source build.

Please consider relaxing this dependency to allow the standard opencv-python package.

See the detailed technical explanation and context in the related issue for fast-plate-ocr: ankandrew/fast-plate-ocr#86

[ankandrew]

See ankandrew/fast-plate-ocr#86 (comment).

[ankandrew]

See ankandrew/fast-plate-ocr#86.