Kivy

Kivy framework

Multiplatform.

• Installation instructions
  • Documentation (link)

Perhaps the most universal multiplatform solution would be to have a web app. See Python, Flask, and WebView blog post (link).

Development

• Examples (link)

UI Designer

• UI designer (docs)

Android

• Kivy on Android docs (link)
• Python for Android project docs (link)
• Package for Android (link)