[tahoe-lafs-trac-stream] [Tahoe-LAFS] #2876: tox won't run on ubuntu xenial: python3

[Tahoe-LAFS]

#2876: tox won't run on ubuntu xenial: python3
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     Reporter:  warner     |      Owner:  daira
         Type:  defect     |     Status:  new
     Priority:  major      |  Milestone:  1.13.0
    Component:  packaging  |    Version:  1.12.1
   Resolution:             |   Keywords:
Launchpad Bug:             |
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Comment (by warner):

 I saw a presentation at !PyBay this last weekend, by one of the Jupyter
 developers, about their transition from py2+py3 to py3-only. He encouraged
 everyone to add a `python_requires=` argument to their `setup()` call,
 which adds metadata that specifies which versions of python the
 package/distribution is compatible with. For their needs, they made sure
 to publish a version that claimed both 2 and 3 before their switchover.
 Later, when a newer version was marked as py3-only, py2-based sites which
 had installed the old version will correctly refuse to upgrade to the
 newer one. He mentioned that older versions of pip and setuptools do not
 pay attention to the `python_requires=` field, and they had some
 workaround and monitoring to keep track of how many installs were
 affected.

 So another approach for us would be to mark our `setup()` with
 `python_requires=`, then remove the manual py3-crowbar from our setup.py.
 I'll do some testing: my hope is that a `pip install` will see the
 metadata and give a useful error (and pip will own that error, not us),
 but when tox3 does `setup.py wheel`, it won't complain.

 If that works, I'll cancel PR-422 and replace it with one that uses
 `python_requires=`.

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Ticket URL: <https://tahoe-lafs.org/trac/tahoe-lafs/ticket/2876#comment:15>
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