bpo-35240: Add "doctest" job to Travis CI

[vstinner]

Create a new "doctest" job in Travis CI to run "make doctest".

https://bugs.python.org/issue35240

[vstinner]

This change:

• make the "cpython" job faster, since it doesn't run "make -C Doc venv doctest" anymore which takes 1 minute
• "make doctest" is now run in parallel
• "make doctest" is now run with a Python compiled in release mode

[zware]

LGTM! This should hopefully make things more stable as well.

[matrixise]

Just one question, why do you specify make -j4, do we have a quadcore?

[vstinner]

@matrixise: last time I checked, Travis CI announced more than 90 cores. Check "pythoninfo" in Travis CI.