Briefcase version metadata

[ForeverWintr]

Hi! I think this is a really cool project, and hope this small contribution will be of use. I intend it to resolve #412 by adding Briefcase-Version to app metadata. It can be retrieved as in your example in the linked issue.

Feedback is welcome.

[freakboy3742]

This looks great - thanks for the contribution! The code and test is great - exactly what I was expecting to see.

All that is missing is some documentation. I'd suggest two things are needed:

1. A new HowTo page for 'Accessing Briefcase packaging metadata at runtime'. This should give a short example for how to access runtime properties (using importlib.metadata); and list the metadata that is available at runtime.

2. A FAQ entry for the specific question that prompted this feature request ("How do I detect if my app is running in a Briefcase-packaged container?") that directs to the HowTo page.

If you're not confident in your tech writing skills, don't worry about that too much - we can always revise the text during the review process.

[ForeverWintr]

Thanks @freakboy3742! I've taken a crack at the documentation, what do you think?

I'm not sure what best practice is for providing backwards compatibility in example code. I've indecisively included both your backwards compatible metadata import, and then printed with f-strings...

[freakboy3742]

Looks great - thanks! The example code you've provided is about as good as we can get for the general case. It might be worth mentioning that the example is "runs on all Pythons" compliant, and can be simplified if you're targetting a specific version of Python (which apps often will be). We also (unfortunately) need to avoid f-strings for now, because we still support Python 3.5.

Don't worry to much about making those changes, though; I'll make a couple of light language edits before land the patch, and I can drop in some extra content about how the example code can be simplified if you're targetting a single Python version.

[codecov]

Codecov Report

Merging #425 into master will increase coverage by 0.00%.
The diff coverage is 100.00%.

Impacted Files	Coverage Δ
src/briefcase/commands/create.py	97.24% <100.00%> (+0.01%)	⬆️

[ForeverWintr]

Great, thanks for the review and edits!