Set start method to forkserver for MacOS

[AlexAndorra]

As discussed in #3912 with @lucianopaz and @twiecki, this PR sets the start method to forkserver when the platform is MacOS. This is intended to solve #3849.

As this is implemented in setup.py, the new start method is modified each time users import PyMC3, but it does it smoothly by using multiprocessing.get_context instead of multiprocessing.set_start_method. From the python docs:

A library which wants to use a particular start method should probably use get_context() to avoid interfering with the choice of the library user.

I did some raw tests locally and this does work!

• No breaking changes (that I know of)
• Added to RELEASE-NOTES.md

[codecov]

Codecov Report

Merging #3919 into master will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master    #3919   +/-   ##
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  Coverage   83.59%   83.59%           
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  Files         103      103           
  Lines       14181    14181           
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  Hits        11854    11854           
  Misses       2327     2327           

[twiecki]

Thanks @AlexAndorra!

[ivlis]

Thank you so much!