Use humanize.intcomma to format years in time module

[carterbox]

Related to #245

Presently, it is hard to read very long times because the largest time denomination that humanize includes is years. In this pull request, humanize.intcomma() is now used to format the number of years returned by naturaldelta() and precisedelta().

Alternate approaches considered include:

1. Adding a millennia denomination, but this would require all translations to be updated.
2. Using the {:n} format specifier along with the standard locale module, but locale has inconsistent behavior across platforms and humanize already uses an internal localization module.

[codecov-commenter]

Codecov Report

Merging #246 (d1faf1c) into master (429e3d8) will increase coverage by 0.02%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master     #246      +/-   ##
==========================================
+ Coverage   97.08%   97.10%   +0.02%     
==========================================
  Files          10       10              
  Lines         685      690       +5     
==========================================
+ Hits          665      670       +5     
  Misses         20       20              

Flag	Coverage Δ
GHA_Ubuntu	95.94% <100.00%> (+0.02%)	⬆️
GHA_Windows	95.65% <100.00%> (+0.03%)	⬆️
GHA_macOS	95.94% <100.00%> (+0.02%)	⬆️

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Impacted Files	Coverage Δ
src/humanize/time.py	96.46% <100.00%> (+0.08%)	⬆️

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[hugovk]

Please could you add tests?

[carterbox]

Yes. Added.

[hugovk]

Thanks!