Add Python information to "datalad wtf" command

[driusan]

datalad wtf did not include any information about which
version of Python is being used. This adds a section to
include the python version number and python implementation
being used to run datalad to aid in debugging.

[yarikoptic]

Who said "elephant"? ;-) Thanks @driusan . FWIW, here is what I see

## python 
  - implementation: CPython
  - python: 2.7.14+

may be change "python" item to "version"? ;)

[codecov]

Codecov Report

Merging #3255 into master will decrease coverage by 0.06%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##           master    #3255      +/-   ##
==========================================
- Coverage   91.04%   90.98%   -0.07%     
==========================================
  Files         263      263              
  Lines       34131    34135       +4     
==========================================
- Hits        31076    31057      -19     
- Misses       3055     3078      +23

Impacted Files	Coverage Δ
datalad/plugin/wtf.py	82.06% <100%> (+0.32%)	⬆️
datalad/downloaders/tests/test_http.py	88.86% <0%> (-3.47%)	⬇️
datalad/downloaders/http.py	82.53% <0%> (-3.18%)	⬇️
datalad/downloaders/base.py	80.72% <0%> (-0.37%)	⬇️

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[driusan]

Sure, I changed the key to "version"