Decorate Pack as dataclass

[HalfWhitt]

With the aliases all defined as class attribute descriptors as of #3213, Pack can be successfully decorated as a dataclass.

The only test that fails-as-is is test_mixin.py::test_constructor, because the auto-genereated dataclass __init__ throws a TypeError rather than a NameError when an erroneous property name is provided. I've changed what the test looks for, and also added a try/except to BaseStyle's fallback __init__ so that the same error is generated under Python 3.9.

Edit: Huh. What are the odds, it's been exactly one year, to the day, since I started this with beeware/travertino#141. I definitely didn't anticipate how long that would take 😄

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

[HalfWhitt]

Decorating Pack as a dataclass attempts to overrides the default <toga.style.pack.Pack object at [memory address]> representation with Pack(name=value, ...). Unfortunately, it tries to do so for all properties, including ones that haven't been set, and aliases which aren't currently valid (which raises an exception). So I've added a BaseStyle.__repr__ that behaves very similarly to the existing __str__, only listing properties that are set. I've also added repr=False to the dataclass arguments to avoid overriding it.

(While I was next to it, I also tweaked __str__ by replacing the generator expression in join() with a list comp.)
Edit: Reverted this change to be consistent with #3213. Also makes it fewer lines in this case. I also made the list-comp-to-generator switch for the new __repr__ method.)

[HalfWhitt]

It's gonna be so satisfying to rip out all this backwards compatibility code once we no longer support Toga < 0.5 and Python 3.9.

[freakboy3742]

Looks good - a fairly minor set of changes at the end of the day. Jeez, why did it take over 12 months to complete such a small PR? 😝 🤣

And yes, the "FINALLY WE GET TO DEPRECATE 3.X" purge is always really satisfying.

[freakboy3742]

I think that was your final blocker for a Toga 0.5 release? Have I missed anything?

[HalfWhitt]

Looks good - a fairly minor set of changes at the end of the day. Jeez, why did it take over 12 months to complete such a small PR? 😝 🤣

Oh, you 🤣

I think that was your final blocker for a Toga 0.5 release? Have I missed anything?

That should finally be it! Let 'er rip!