Update minimum_unit handling of naturaldelta and naturaltime

[hugovk]

Fixes #75 (comment).

Changes proposed in this pull request:

• Upgrade the unit for naturaldelta or naturaltime, even if minimum_unit is lower

TODO

• Update docstrings
• Should naturaldelta(delta=FOUR_MICROSECONDS, minimum_unit="milliseconds") return "a moment" instead of "0 milliseconds"? (And similarly for naturaltime)

[codecov-commenter]

Codecov Report

Merging #142 into master will not change coverage.
The diff coverage is 100.00%.

@@            Coverage Diff            @@
##            master      #142   +/-   ##
=========================================
  Coverage   100.00%   100.00%           
=========================================
  Files            9         9           
  Lines          554       556    +2     
=========================================
+ Hits           554       556    +2     

Flag	Coverage Δ
#GHA_Ubuntu	100.00% <100.00%> (ø)
#GHA_Windows	100.00% <100.00%> (ø)
#GHA_macOS	100.00% <100.00%> (ø)

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Impacted Files	Coverage Δ
src/humanize/time.py	100.00% <100.00%> (ø)
tests/test_time.py	100.00% <100.00%> (ø)

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[hugovk]

• Should naturaldelta(delta=FOUR_MICROSECONDS, minimum_unit="milliseconds") return "a moment" instead of "0 milliseconds"? (And similarly for naturaltime)

Leaving existing behaviour for now.