Fix brew symlinking in CI

[ulupo]

Reference issues/PRs Vaguely related to #527, #514, #513.

Types of changes

• [x] Bug fix (non-breaking change which fixes an issue)
• [ ] New feature (non-breaking change which adds functionality)
• [ ] Breaking change (fix or feature that would cause existing functionality to change)

Description The macOS Azure pipelines have beginning to fail as brew fails to create symlinks when upgrading python. This appears identical to the issue opened in https://github.com/actions/virtual-environments/issues/2322 (GitHub actions). As a hotfix, the line rm '/usr/local/bin/2to3' has been added. Furthermore, a brew upgrade step has been reinstated. However, this also initially failed due to another symlink issue on gcc@8 and gcc@9, as described in https://github.com/actions/virtual-environments/issues/2391. As a hotfix to this, the line brew unlink gcc@8 && brew unlink gcc@9 has been added.

The status of the causing issue should be monitored and this hotfix might become unnecessary in the future.

Checklist

• [x] I have read the guidelines for contributing.
• [x] My code follows the code style of this project. I used flake8 to check my Python changes.
• [ ] My change requires a change to the documentation.
• [ ] I have updated the documentation accordingly.
• [ ] I have added tests to cover my changes.
• [x] All new and existing tests passed. I used pytest to check this on Python tests.