Use highest possible pickle protocol

[micbou]

Currently, Jedi uses version 2 of the pickle protocol to be compatible with all supported versions of Python. However, we can be smarter and use the highest version of the pickle protocol that is compatible with the version of Python running Jedi itself and the one running the environment. Examples:

• if both Jedi and the environment are running under Python 3.6, use version 4 of the protocol;
• if Jedi is running under Python 3.6 but the environment is Python 3.3, use version 3;
• if Jedi is running under Python 3.6 but the environment is Python 2.7, use version 2;
• etc.

The reason for using the highest possible version of the pickle protocol is that performance is better with newer versions of the protocol as shown by the following results:

Interpreter	Python 2.7				Python 3.6
Environment	Python 2.7		Python 3.6		Python 2.7		Python 3.6
Pickle protocol	Before	After	Before	After	Before	After	Before	After
	2	2	2	2	2	2	2	4
os	0.153s	0.159s	0.185s	0.184s	0.0978s	0.105s	0.074s	0.0714s
numpy	0.271s	0.26s	0.262s	0.259s	0.206s	0.198s	0.188s	0.173s
cv2	0.964s	0.949s	1.08s	1.07s	0.513s	0.513s	0.465s	0.308s

Measurements obtained with this script when completing the os, numpy, and cv2 (opencv-python) modules on Ubuntu 18.04 64-bit.

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This change is 

[codecov-io]

Codecov Report

Merging #1152 into master will decrease coverage by 0.01%.
The diff coverage is 77.77%.

@@            Coverage Diff             @@
##           master    #1152      +/-   ##
==========================================
- Coverage    92.7%   92.69%   -0.02%     
==========================================
  Files          57       57              
  Lines        6910     6912       +2     
==========================================
+ Hits         6406     6407       +1     
- Misses        504      505       +1

Impacted Files	Coverage Δ
jedi/api/environment.py	78.53% <100%> (ø)	⬆️
jedi/evaluate/compiled/subprocess/__init__.py	74.75% <75%> (-0.25%)	⬇️

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[davidhalter]

I'm not that happy with the added complexity, but I can understand the cause. Do you think it's actually worth it? I know it feels like quite a performance increase for cv2 on Python3.6, but it's also just that. I guess I'll leave the decision if I should merge to you.

[blueyed]

Interesting!
highest_pickle_protocol should have some unit tests probably.

[micbou]

I'm not that happy with the added complexity, but I can understand the cause. Do you think it's actually worth it? I know it feels like quite a performance increase for cv2 on Python3.6, but it's also just that. I guess I'll leave the decision if I should merge to you.

I think it's worth. All modules with huge files are going to benefit from this and not only on Python 3.6 but all versions of Python starting from 3.3.

highest_pickle_protocol should have some unit tests probably.

Added a bunch of unit tests.

[davidhalter]

Fair enough.