default to python3 on Unix

[stevengj]

Now that Conda defaults to Python 3 (JuliaPy/Conda.jl#108), to be consistent PyCall should default to python3 on non-Mac Unices.

Python 2 is still supported, you just need to configure PyCall with ENV["PYTHON"]="python2" or similar. Existing installations will not be affected because PyCall remembers what Python you last configured with.

See also #435.

[codecov-io]

Codecov Report

Merging #512 into master will not change coverage.
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  Coverage   65.55%   65.55%           
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  Files          19       19           
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  Hits          986      986           
  Misses        518      518

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[PallHaraldsson]

I see you took my advice, but only partly (Subject ".. on Unix" [too, not only] threw me off, didn't realize Conda applied for PyCall; UTF-8 stuff still applies here).

Since you did, should Python 3.7 be preferred (and at least tested)? At least this feature:

https://www.python.org/downloads/release/python-370/
"Avoiding the use of ASCII as a default text encoding (PEP 538, legacy C locale coercion and PEP 540, forced UTF-8 runtime mode)", i.e. from PEP 540: "Add the -X utf8 command line option and PYTHONUTF8 environment variable to control UTF-8 Mode."

I'm not sure, but I think this "forced UTF-8 runtime mode" may align better with Julia. At least if Python 3.7 is used, maybe it should be enabled by default?

It may or may not (already) be backported to earlier versions:
https://pythoninsider.blogspot.com/2018/08/python-356-and-python-349-are-now.html

--
I'm a little skeptical of leaving macOS at Python2 (in some cases it may not matter, then could have stayed with Python2 on Linux, but in other cases code will cork on one but not the other platform).

Should at least Windows also default to Python3? It usually has no Python installed, so if conda gets one for you it may as well be Python 3.7.

[stevengj]

MacOS and Windows default to using Conda, which now defaults to Python 3.

[stevengj]

It might be reasonable to set UTF-8 I/O in libpython, but not by an environment variable (which would affect child processes too). We need to find an API for this.

(When pyjulia is used, we shouldn't change the I/O mode, however.)