Do not issue deprecation warnings during import

[samueljsb]

Prior to this change, the Unit enum would issue a deprecation warning
during import when its __dict__ was accessed. This is not correct: the
warning should only be issued if the enum is actually accessed.

This change ensures that no deprecation warnings are issued by the enum
during import.

Fixes #242.

e.g:

$ python -Werror
>>> from humanize import time
>>> time.Unit.DAYS
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
  File "/Users/samuelsearles-bryant/dev/samueljsb/humanize/src/humanize/time.py", line 50, in __getattribute__
    warnings.warn(
DeprecationWarning: `Unit` has been deprecated. The enum is still available as the private member `_Unit`.

[samueljsb]

@hugovk This fixes the reported bug, although it's a bit hacky.

I can't find any good advice for deprecating enums anywhere online.

I also considered checking whether the name began with __ to avoid issuing a warning when we access magic methods. This fails because Python also gets the mro() during import, so we'd have to ignore that too... and that becomes a game of whack-a-mole (and no less hacky).

[hugovk]

Thanks for checking!

How do you feel about this hacky solution?

One option is just to skip this one particular warning. The enum was added fairly recently (#104) so perhaps it's not being used.

[samueljsb]

I think we want something in __getattribute__ - if anyone does happen to be using it then .-access is the most likely way they'll be getting at its members.

If we're going to remove the deprecation warnings (and the public aliases) soon then I think this is an acceptable short-term hack.

[hugovk]

Sounds reasonable. Please could you check the CI failures?

[hugovk]

This is being rather tricky for limited benefit, so on balance let's just keep Unit and deprecate the rest. If you're busy I can have a look at this so we can get the release out. Thanks for all your work so far!

[hugovk]

Please see PR #252 to keep Unit.

[samueljsb]

Hey sorry for disappearing on you! Thanks for handling this - I agree the enum is to much hassle to deprecate cleanly.

[hugovk]

All good!

The other deprecations have been released in 3.14.0. I'll give it a bit of time and then release 4.0.0 with removals.