Moved pymanopt tests to experimental test group #2138

[daviddavo]

Description

Pymanopt is an extra dep, but its tests are run among other dependencies

Related Issues

• 2138

References

Checklist:

• [x] I have followed the contribution guidelines and code style for this project.
• [x] I have added tests covering my contributions.
• [ ] I have updated the documentation accordingly.
• [x] I have signed the commits, e.g. git commit -s -m "your commit message".
• [x] This PR is being made to staging branch AND NOT TO main branch.