Perform attr order checks after field transformer.

[jamesmurphy-mc]

Fixes: #1147

Summary

Previously, attrs performed order checks for mandatory vs non-mandatory attributes before applying the field transformer, giving errors even when the field transformer intended to fix such errors such as by supplying defaults, factories, making args keyword-only, removing attrs, or reordering attrs.

This PR moves the checks after the field transformer.

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[sscherfke]

Good idea and looks good. :)

[jamesmurphy-mc]

Thanks! I just merged the latest autocommit from main into the PR so assuming all checks pass, I think it's ready to merge.

[hynek]

Thanks!

Great to meet a fellow Python YouTuber with checks notes 100x subscribers. :D