[Distutils] python_version>="2.7" environment marker difficulties

[Michael R. Crusoe]

Hello,

To support a dependency[0] which has a different name for its Py2 & Py3
versions, our program schema_salad uses the "python_version" environment
marker.

This seems to not be supported when users have older setuptools installed[1]

With setuptools before version 17.1 one gets

error in schema-salad setup command: Invalid environment marker:
python_version>="2.7"

I tried to bundle a newer version with ez_setup.py but that isn't working
either:
https://github.com/common-workflow-language/schema_salad/commit/04a956dbef40b32ef9c486ddab4dcff544dc2c65#commitcomment-16745845

"ez_setup() bootstraps transparently if you're using python setup.py
install but not for pip install of the same package (where it complains
that you need to upgrade setuptools first)"

Code is at
https://github.com/common-workflow-language/schema_salad/blob/envmarker-test/setup.py#L45

Is there a better way to deal with the divergent names for our dependency?

Thanks in advance,

[0] https://pypi.python.org/pypi/avro vs
https://pypi.python.org/pypi/avro-python3
[1]
https://github.com/common-workflow-language/schema_salad/issues/17#issue-141147041

-- 
Michael R. Crusoe     CWL Community Engineer     crusoe at ucdavis.edu
Common Workflow Language project    University of California, Davis
https://impactstory.org/MichaelRCrusoe http://twitter.com/biocrusoe
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