Fix bug in check for EU country set

[orhoj]

Purpose

Fix a bug in the check for EU country set.

Changes

• Invert the check to actually check if the set of countries is exactly the EU members.

Checklist

• My code follows the style of this project.
• The code compiles without warnings.
• I have performed a self-review of the changes.
• I have documented my code, in particular the intent of the
  hard-to-understand areas.
• (If necessary) I have updated the CHANGELOG.