Change Catalog __repr__ to sorted set

[MaxSchambach]

Description

Change the representation of Catalog to a set of sources keys sorted alphabetically.

The sorted representation is case-insensitive. This way, looking up keys in a catalog becomes a little easier.

Fixes # issue

Type of change

• 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 not work as expected)
• Documentation update
• Refactoring including code style reformatting
• Other (please describe):

Checklist:

• I have read the contributing guideline doc (external contributors only)
• Lint and unit tests pass locally with my changes
• I have kept the PR small so that it can be easily reviewed
• I have made corresponding changes to the documentation
• I have added tests that prove my fix is effective or that my feature works
• All dependency changes have been reflected in the pip requirement files.

[pzdkn]

LGTM!