Updates for Python 3.14

[janezd]

Issue

• Gensim doesn't work on Python 3.14
• Tests with Qt6 detect an error in the Groups widget that went undetected in Qt 5.

Description of changes

• Skip installation on Python 3.14, and disable the widget. Add a test that will remind us to periodically check for gensim updates.
• Fix the error in Group: weights were modified in-place instead of in a copy.

Includes

• Code changes
• Tests