Allow wheels to determine the required dependencies

[MinchinWeb]

This allows universal wheels to be generated and reliably installed.

Background

Version 0.11 (of arrow) introduced caching, but based on a feature that was added to the Python standard library post-2.7. This meant that arrow became un-usable with Python 2.7.

Version 0.12 was released to fix the above problem. It did this by adding the following code to setup.py:

install_requires = ['python-dateutil']
if sys.version_info[0] < 3:
    install_requires.append('backports.functools_lru_cache==1.2.1')

This code declares the required dependency, but requires code to run on install to do so. Wheels do not run code at install time, rather code is run on wheel generation. This meant that generated wheels would either always require this backports package (if the wheel was generated by a system running Python 2.7) or never require it (if the wheel was generated by a system running Python 3). Even though arrow does not provide wheels on PyPI, this can still cause issues. This is because pip, on end user machines, will generate and cache a wheel when downloading source distributions (like arrow provides), and this pip cache, and thus the generated wheel, is shared by all version of Python installed on the end user's machine, and used for later installs.

Solution

This PR declares the required dependency in a conditional way that wheel can read it on install.

Further Reading

• Conditional Python Dependencies
• Wheel Documentation -- Defining conditional dependencies

[MinchinWeb]

This also address #497, #499

[codecov-io]

Codecov Report

Merging #504 into master will not change coverage.
The diff coverage is n/a.

@@          Coverage Diff          @@
##           master   #504   +/-   ##
=====================================
  Coverage     100%   100%           
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  Files          14     14           
  Lines        3189   3189           
  Branches      229    229           
=====================================
  Hits         3189   3189

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[Gr1N]

We really need this PR, I have so much pain with my builds on CI now...

@crsmithdev could you please take a look?

[ramonsaraiva]

Could you just fix this alignment? Just remove 4 spaces, everything else is LGTM.

[MinchinWeb]

@ramonsaraiva updated (I believe) as you requested! Everything should be good to go!

[ramonsaraiva]

LGTM

[Gr1N]

@ramonsaraiva, could you please upload new version of package with fix? There are too many people (#495) which are waiting for new version.