Make Ukrainian locale name follow ISO 639, update it and add tests

[nykula]

Pull Request Check List

• Added tests for changed code.
• Updated documentation for changed code.

Fixes#955.

[codspeed-hq]

Merging this PR will not alter performance

✅ 1 untouched benchmark

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_{Comparing nykula:master (fd45ad8) with master (cbab60c)}

[afastronomer]

d = pendulum.now().subtract(weeks=5)
assert d.diff_for_humans(locale=locale) == "5 тижнів тому"

[nykula]

weeks=5 behaves in the same way as months=1, resulting in 1 місяць тому. Should I add that assertion?

[ashb]

What do other locales do here?

[nykula]

All locales behave the same in this regard. @afastronomer most likely meant that from the grammar point of view, 2 weeks and 5 weeks correspond to different forms in Ukrainian. But the latter form is currently unused in Pendulum; there's no feature, that I know of, to get a human representation of a duration in a specific unit such as weeks. One way to get to the form is:

pendulum.locale('uk').translation(f"units.week.{pendulum.locale('uk').plural(5)}").format(5)
# -> '5 тижнів'

[nykula]

My bad, left the updated test file uncommitted.