Don't deprecate time.Unit enumeration

[hugovk]

Replaces and closes #244.

Changes proposed in this pull request:

• Let's keep the Unit enum after all, it's being a bit troublesome to deprecate
• And, unlike the function deprecations (Deprecate private functions #234), it's not so important to keep this internal
• We could expose this as available in a followup (Expose public functions from the root package #226)

[codecov-commenter]

Codecov Report

Merging #252 (1bedec1) into master (35201aa) will increase coverage by 0.54%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master     #252      +/-   ##
==========================================
+ Coverage   97.10%   97.65%   +0.54%     
==========================================
  Files          10       10              
  Lines         692      681      -11     
==========================================
- Hits          672      665       -7     
+ Misses         20       16       -4     

Flag	Coverage Δ
GHA_Ubuntu	96.47% <100.00%> (+0.52%)	⬆️
GHA_Windows	96.18% <100.00%> (+0.51%)	⬆️
GHA_macOS	96.47% <100.00%> (+0.52%)	⬆️

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Impacted Files	Coverage Δ
src/humanize/time.py	98.15% <100.00%> (+1.66%)	⬆️
tests/test_time.py	100.00% <100.00%> (ø)

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