naturalsize() ignores the locale decimal separator #460

On current main, with a locale whose decimal separator is a comma:

>>> import humanize
>>> humanize.i18n.activate("fr_FR")
>>> humanize.intcomma(1.5)
'1,5'
>>> humanize.naturalsize(1_500_000)
'1.5 Mo'

The same happens for every locale with "," as the decimal separator (de_DE, fr_FR, hu_HU, it_IT, lv, pt_BR). intword() and intcomma() use decimal_separator(), but naturalsize() does not.

#378 mentioned that naturalsize was left untouched as "the one loose end", and #357 got a ~10% CodSpeed regression on test_naturalsize when it tried to fix this.

Proposal: use decimal_separator() in naturalsize(), but skip the extra work when the separator is "." so the default path is not slowed down, and check the result against the existing benchmarks.

Open PRs #403, #457 and #367 also touch naturalsize(). Would you prefer us to wait for them, or go ahead and rebase later?