Revert using highest possible pickle protocol

[micbou]

See discussion in PR #1108. Add a note that the version of the pickle protocol should be increased to 4 when Python 2.7 and 3.3 are dropped.

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This change is 

[davidhalter]

@blueyed What do you think? I think my comments in #1108 still apply.

[codecov-io]

Codecov Report

Merging #1164 into master will increase coverage by 0.01%.
The diff coverage is 85.71%.

@@            Coverage Diff             @@
##           master    #1164      +/-   ##
==========================================
+ Coverage    92.6%   92.61%   +0.01%     
==========================================
  Files          57       57              
  Lines        6884     6882       -2     
==========================================
- Hits         6375     6374       -1     
+ Misses        509      508       -1

Impacted Files	Coverage Δ
jedi/api/environment.py	78.53% <100%> (ø)	⬆️
jedi/evaluate/compiled/subprocess/__init__.py	75% <83.33%> (+0.24%)	⬆️

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[blueyed]

I would say to keep it given that my plan work (#1108 (comment)).
IIRC the problem was to have the highest_pickle_protocol function available in the client - that's when I've started doing the PYTHONPATH PR (#1159).

[davidhalter]

I'll just leave this one open, let's see what's going to happen.

[davidhalter]

Since @blueyed found a solution for the problem in #1108, I think we can safely close this one.