[PR #13263/ec870850 backport][8.3.x] Fix prepare-release-pr script

[patchback]

This is a backport of PR #13263 as merged into main (ec87085).

The script is currently failing with:

remote: Support for password authentication was removed on August 13, 2021.
remote: Please see https://docs.github.com/get-started/getting-started-with-git/about-remote-repositories#cloning-with-https-urls for information on currently recommended modes of authentication.
fatal: Authentication failed for 'https://github.com/pytest-dev/pytest.git/'

Decided to remove the usage of github3 and use the gh command-line tool directly, which simplifies the script, integrates nicely with GH, and enables to run it locally seamlessly.