Pygame-menu

- Fixed assertion error on custom theme, 196.

This version introduces a lot of changes from v2:
- New documentation page: https://pygame-menu.readthedocs.io.
- Scroll support.
- Image support, widget and background.
- Added themes.
- As the library has been rewritten, many changes have to be made from v2 to v3. See migration guide.

Thanks to:
- anxuae
- eforgacs

[Migration guide](https://pygame-menu.readthedocs.io/en/latest/_source/migration_guide.html)

- Clicking outside the selected widget disables active status.

- Fix exit, 251.
- Docs improvements.

<a href="https://github.com/ppizarror/pygame-menu/blob/v2/README.md">Documentation</a>

**NOTE: pygame-menu *v2* will not provide new widgets or functionalities, consider upgrading to the latest version.**

- Make setup universal, 170.

<a href="https://github.com/ppizarror/pygame-menu/blob/v2/README.md">Documentation</a>

- Update README and documentation.
- Fix position calculation for python 2.

<a href="https://github.com/ppizarror/pygame-menu/blob/v2/README.md">Documentation</a>