Reinstate VERSION and deprecate

[hugovk]

Changes proposed in this pull request:

• VERSION was inadvertently removed in Use importlib.metadata to get package version instead of pkg_resources.get_distribution to decrease memory consumption #227 but not yet released
• Let's reinstate it
• But as it's non-standard and we've had the standard __version__ since humanize 1.0 (Feb 2020), let's deprecate it and remove it in the next major bump
• We can use the technique for deprecating constants only available in Python 3.7+, which is fine as the next major bump will also drop 3.6 (Drop support for Python 3.6 #239)

[codecov-commenter]

Codecov Report

Merging #240 (710ca33) into master (374f14e) will increase coverage by 0.00%.
The diff coverage is 100.00%.

@@           Coverage Diff           @@
##           master     #240   +/-   ##
=======================================
  Coverage   99.68%   99.69%           
=======================================
  Files           9       10    +1     
  Lines         635      651   +16     
=======================================
+ Hits          633      649   +16     
  Misses          2        2           

Flag	Coverage Δ
GHA_Ubuntu	98.46% <100.00%> (+0.03%)	⬆️
GHA_Windows	98.15% <100.00%> (+0.04%)	⬆️
GHA_macOS	98.46% <100.00%> (+0.03%)	⬆️

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Impacted Files	Coverage Δ
tests/test_i18n.py	100.00% <ø> (ø)
tests/test_number.py	100.00% <ø> (ø)
tests/test_time.py	100.00% <ø> (ø)
src/humanize/__init__.py	100.00% <100.00%> (ø)
tests/test_humanize.py	100.00% <100.00%> (ø)

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