adds multi-OS support to travis CI

[mastersplinter]

Adds multi-OS Support to address #109

This requires a few changes to the current approach:

• removing linux/macOS specific environment setup
• changing the "language" feature because "Python" is currently not supported for Windows instances on Travis, using environment variables instead
• separating the installation of conda, and the setup of the conda environment to keep the CI file easy to read

[codecov-io]

Codecov Report

Merging #124 into master will decrease coverage by 0.07%.
The diff coverage is n/a.

@@            Coverage Diff            @@
##           master    #124      +/-   ##
=========================================
- Coverage   95.98%   95.9%   -0.08%     
=========================================
  Files           9       8       -1     
  Lines         448     440       -8     
=========================================
- Hits          430     422       -8     
  Misses         18      18

Impacted Files	Coverage Δ
pandera/__init__.py

---

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[cosmicBboy]

not sure why this build is failing:
https://travis-ci.org/pandera-dev/pandera/jobs/616075868#L418-L419

[mastersplinter]

Please consider this PR WIP - I need to work through the test failures.

[mastersplinter]

@cosmicBboy please re-review, I've had help from @bensaxby to resolve the test failures.

This PR extends CI support from Linux to also include Windows and osX/macOS by:

• handling the way Python and Conda work on Travis for different OS versions
• includes code like conda list because it's easier to debug the CI if it's included
• contains a specific safety net for if the terminal window is sized too small. The setting of the doctest_global_setup variable is included because when Travis is running a Python environment on macOS it misreports the size of the terminal window as 0 cols, 0 rows. This can be replicated by running make doctest locally with a small terminal window. When the window is too small Pandas trims the dataframe output with ellipsis. I've been unable to replicate this bug locally, i.e. terminal always reports the correct size, so I think it's a bug in Travis-CI.
• the intermittent Python 3.5 error was caused by a Travis CI cache issue.

Separate to enabling multi-OS support, I believe we should specify specific environments and packages as is achieved here:
https://github.com/kangwonlee/travis-yml-conda-posix-nt/blob/master/.travis.yml

i.e. we would adopt:

env:
  - CONDA_PYTHON=3.6
  - CONDA_PYTHON=3.7
  - CONDA_PYTHON=2018.12
  - CONDA_PYTHON=2019.03
  - CONDA_PYTHON=2019.07
  - CONDA_PYTHON=nightly

and those get substituted into a specific conda environment.yaml file containing explicitly tested dependencies.

At the moment, because we only specify the "minimum" package versions, when the conda environment is rebuilt on CI runs, there's a risk that a change in a new version of a dependency would break the CI Pipeline.

[mastersplinter]

I think Codecov is misreporting the coverage issue and can be bypassed/ignored.

[cosmicBboy]

@mastersplinter looking good to me!

looks like flake8 is complaining:

tests/test_docs_setting_column_widths.py:4:1: E302 expected 2 blank lines, found 1
tests/test_docs_setting_column_widths.py:9:37: E711 comparison to None should be 'if cond is None:'
tests/test_docs_setting_column_widths.py:10:37: E711 comparison to None should be 'if cond is None:'
tests/test_docs_setting_column_widths.py:10:44: W292 no newline at end of file

[cosmicBboy]

will ignore code coverage drop, looks like a non-issue

[mastersplinter]

sorry about that - flake8 compliance was a silly miss on my part - I've updated.

FYI I've opened a bug report on the TravisCI Community documenting the specific issue with the way Python on macOS images on Travis report their terminal width.

@cosmicBboy should be good to go