RF: Require Python >=3.6

[mih]

PY35 will come to its end-of-life (EOL) in a week from now.

https://devguide.python.org/#status-of-python-branches

Fixes gh-4878

TODO

• Can more travis runs be stripped?
• Scan the code base to remove PY35 compatibility kludges

[codecov]

Codecov Report

Merging #4879 into master will increase coverage by 0.35%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master    #4879      +/-   ##
==========================================
+ Coverage   89.36%   89.72%   +0.35%     
==========================================
  Files         289      289              
  Lines       39587    40655    +1068     
==========================================
+ Hits        35377    36477    +1100     
+ Misses       4210     4178      -32     

Impacted Files	Coverage Δ
datalad/utils.py	82.34% <100.00%> (+0.19%)	⬆️
datalad/support/tests/test_vcr_.py	57.89% <0.00%> (-3.22%)	⬇️
datalad/interface/tests/test_unlock.py	95.87% <0.00%> (-1.98%)	⬇️
datalad/distributed/tests/ria_utils.py	44.73% <0.00%> (-1.21%)	⬇️
datalad/support/vcr_.py	49.12% <0.00%> (-0.88%)	⬇️
datalad/tests/utils.py	87.15% <0.00%> (-0.15%)	⬇️
datalad/support/digests.py	100.00% <0.00%> (ø)
datalad/tests/test_base.py	100.00% <0.00%> (ø)
datalad/downloaders/shub.py	100.00% <0.00%> (ø)
datalad/tests/test_config.py	100.00% <0.00%> (ø)
... and 155 more

---

Continue to review full report at Codecov.

Legend - Click here to learn more
Δ = absolute <relative> (impact), ø = not affected, ? = missing data
Powered by Codecov. Last update 105e1ff...b0cf49c. Read the comment docs.

[mih]

I thought we talk about it in the call tomorrow and weigh the pros and cons... Only pros then ;-)

🏃🏂🏄⛷️🏇