Fix rollover from ZB to 1.0 YB

[hugovk]

Fixes #205.

Avoid the conversion to float where possible.

Python 3.13.0rc2 (v3.13.0rc2:ec610069637, Sep  6 2024, 17:57:29) [Clang 15.0.0 (clang-1500.3.9.4)] on darwin
Type "help", "copyright", "credits" or "license" for more information.
>>> from humanize import filesize
>>> filesize.naturalsize(100_000_000_000_000_000_000_000)
'100.0 ZB'
>>> filesize.naturalsize(1_000_000_000_000_000_000_000_000)
'1.0 YB'
>>> filesize.naturalsize(10_000_000_000_000_000_000_000_000)
'10.0 YB'
>>> filesize.naturalsize(1_000_000_000_000_000_050_331_648)
'1.0 YB'
>>> filesize.naturalsize(1_000_000_000_000_000_050_331_649)
'1.0 YB'

[asmacdo]

Looks good to me, thanks!