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ROS2 is comfortably using Python3, so let that alleviate your concerns.

The error you see wnen using rosdep install is just rosdep telling you that it doesn't know what "opencv2" is for your given OS and so it can't install something that satisfies the requirement listed in the package.xml of your cv_basics pacakge. As for resolutions, I'll refer you to question #232795 which does a good job of explaining how rosdep works and what you can do next.

If you've installed OpenCV2 yourself and have confirmed that the version you've installed matches the version expected by whichever package has the dependency, then you should be okay to ignore the error. You can ignore errors when by adding -r to your rosdep call. This allows rosdep to continue installing dependencies, despite errors.

If you'd rather fix the error, then you probably want to change "opencv2" in your package.xml to "opencv-python" so that rosdep can find it. opencv-python points to OpenCV 4.5.3.56 so it should be what you're looking for.