Restrict gmsh version to <4.7.

What does this MR do?

Restrict the version of the gmsh Python module to <4.7. The currently available version 4.7 cannot be installed with Python 3.8.

Is there something that needs to be double checked?

No.

Can this MR be accepted?

• Implemented fix
• [ ] Added/Updated documentation
• Pipeline passing
• Squash option set
• Delete branch option set
• Added entry to CHANGELOG.md

Assignee: If the Squash option is set, check/update the commit message right before merging!

Related issues

Closes #