bpo-35240: Travis CI runs doctest without XVFB_RUN

[vstinner]

https://bugs.python.org/issue35240

[vstinner]

I don't see the point of running "make doctest" with xvfb-run.

I guess that it was copied/pasted when "make doctest" has been added to Travis CI by @matrixise in .travis.yml: commit 859c068.

To run the Python test suite, xvfb-run makes sense since multiple tests run a GUI.

[vstinner]

Oh, doctest failed on Travis CI :-(

Warning, treated as error:
**********************************************************************
Line 12, in default (setup code)
Failed example:
    from turtle import *
    turtle = Turtle()
Exception raised:
    Traceback (most recent call last):

[JulienPalard]

The current make doctest needs an X server to test turtle so this PR is currently failing with:

    _tkinter.TclError: no display name and no $DISPLAY environment variable

But this can be gracefully muted, like we're currently disabling those same tests when we can't import tkinter, in Docs/config.py check for:

doctest_global_setup = '''
try:
    import _tkinter
except ImportError:
    _tkinter = None
'''

[vstinner]

I was wrong. PR #10753 seems to be a better option.