www/py-nicegui: Update to 2.20.0

• Remove devel/py-wait-for2 dependency (no needed anymore since 2.20.0).

ChangeLogs:

• https://github.com/zauberzeug/nicegui/releases/tag/v2.19.0
• https://github.com/zauberzeug/nicegui/releases/tag/v2.20.0

Approved by: acm (mentor)