Do not strip libraries from PyPI packages

[rmartin16]

Changes

• We've had issues with stripping libraries from PyPI before; this disables for Flatpak builds.
• This is noticeably a problem for PySide2...although, PySide6 seemingly survives the stripping....either way, it seems a better policy to leave these libraries as-is

PR Checklist:

• All new features have been tested
• All new features have been documented
• I have read the CONTRIBUTING.md file
• I will abide by the code of conduct

[freakboy3742]

Definitely makes sense to me.

However, we should probably add a release note to Briefcase to describe this. It's enough of a change that it would conceivably be surprising to someone.

Marking as approved; won't merge until we've got a changenote PR in place.