add python3 support

[Thomas Gilgenast]

python 2 is dying soon, and as lib5c becomes used by more client libraries and users, they might want to install python3-only packages next to lib5c

this seems like more work than we initially expected

one radical viewpoint could be to support only python3 for new versions of lib5c (an increasingly popular strategy)

[Thomas Gilgenast]

Removing milestone: 1.0.0 (automated comment)

[Thomas Gilgenast]

while this is technically challenging, it has become significantly more important now that python 2 has passed it’s end-of-life

an example of the issues this is beginning to cause is the issue fixed by 79c5a8c - the issue was that because statsmodels no longer uploads python 2 wheels to pypi, a simple pip install lib5c began crashing on windows when build dependencies were missing and caused the build to fail. (our policy is to support pip install lib5c on all platforms with no additional dependencies or configuration other than having an up-to-date version of pip.)

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[Thomas Gilgenast]

we have a lot of progress on this in this branch: https://bitbucket.org/creminslab/lib5c/branch/py3-modernize

currently tests are passing in both py27 and py37

test coverage is not complete of course so there may be many py3 bugs to find

we plan to release “experimental” py3 support soon (just need to wire tox into bitbucket pipelines)

[Thomas Gilgenast]

experimental py3 support is now available in a pre-release version 0.5.6.a2 while we collect information on what low-coverage code paths might be broken in py3

get it with pip install lib5c==0.5.6.a2

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[Thomas Gilgenast]

this is now merged to master and released as 0.6.0 as of 50c03ab