[FIX] Explicitly define the protocol version for pickling

[lanzagar]

Issue

Data saved in pickle format in python 3.8 was not readable on older versions of python.

Description of changes

Define a new constant PICKLE_PROTOCOL and fix it to a specific version (4, as was used now on py3.6 & py3.7). Use it instead of pickle.HIGHEST_PROTOCOL, which became 5 in py3.8.

Disclaimer: I also removed from .io_base import * in data/__init__.py as we don't need to make the base classes publicly visible in a top level module. Orange.data even had Flags imported twice into its namespace, since it was provided by io_base and later overwritten by from .io import *

Includes

• Code changes
• Tests
• Documentation

[codecov]

Codecov Report

Merging #4388 into master will increase coverage by <.01%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##           master    #4388      +/-   ##
==========================================
+ Coverage   87.13%   87.13%   +<.01%     
==========================================
  Files         399      399              
  Lines       72960    72961       +1     
==========================================
+ Hits        63577    63578       +1     
  Misses       9383     9383