De-duplicate Application URL options during project conversion

[rmartin16]

Describe the bug

I tried to convert my qbittorrentui app and was presented with duplicate options for app URL.

-- Application URL -----------------------------------------------------------

What website URL do you want to use for this application? The following URLs
are defined in your existing 'pyproject.toml'; select 'Other' to provide a
different URL.

  1) https://github.com/rmartin16/qbittorrentui
  2) https://github.com/rmartin16/qbittorrentui
  3) Other

Application URL [1]: 

Steps to reproduce

Clone https://github.com/rmartin16/qbittorrentui/ and run briefcase convert

Expected behavior

De-dup these options.

Screenshots

No response

Environment

• Operating System: any
• Python version: any
• Software versions:
  • Briefcase: 0.3.21.dev120+gec9fbb4a

Logs

No response

Additional context

No response

[freakboy3742]

The other potential fix for this bug - move to a multi-valued URL definition that would be consistent with PEP 621. The same also goes for Author and Maintainer fields. My inclination is that this would be a better long term approach, as it aligns with the long term goal of migrating to PEP 621 formats for common metadata.