Fix MacOS py37 on Travis

[nicoddemus]

This has been failing as of 2018-10-23 while installing gcc with
this message:

	==> Installing numpy dependency: gcc
	==> Downloading https://homebrew.bintray.com/bottles/gcc-8.2.0.high_sierra.bottl
	######################################################################## 100.0%
	==> Pouring gcc-8.2.0.high_sierra.bottle.1.tar.gz
	Error: The `brew link` step did not complete successfully
	The formula built, but is not symlinked into /usr/local
	Could not symlink include/c++
	Target /usr/local/include/c++
	already exists. You may want to remove it:
	  rm '/usr/local/include/c++'
	To force the link and overwrite all conflicting files:
	  brew link --overwrite gcc
	To list all files that would be deleted:
	  brew link --overwrite --dry-run gcc
	Possible conflicting files are:
	/usr/local/include/c++ -> /usr/local/Caskroom/oclint/0.13.1,17.4.0/oclint-0.13.1/include/c++

Example failure: https://travis-ci.org/pytest-dev/pytest/jobs/444989259

[codecov]

Codecov Report

Merging #4217 into master will decrease coverage by 1.07%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master    #4217      +/-   ##
==========================================
- Coverage   95.81%   94.73%   -1.08%     
==========================================
  Files         109      109              
  Lines       24295    24378      +83     
  Branches     2395     2399       +4     
==========================================
- Hits        23278    23095     -183     
- Misses        721      988     +267     
+ Partials      296      295       -1

Flag	Coverage Δ
#docs	28.51% <ø> (ø)	⬆️
#doctesting	28.51% <ø> (ø)	⬆️
#linting	28.51% <ø> (ø)	⬆️
#linux	93.75% <ø> (-1.85%)	⬇️
#nobyte	91.32% <ø> (-0.57%)	⬇️
#numpy	91.58% <ø> (-1.32%)	⬇️
#pexpect	?
#py27	92.85% <ø> (-1.11%)	⬇️
#py34	91.58% <ø> (-0.62%)	⬇️
#py35	91.59% <ø> (-0.62%)	⬇️
#py36	92.33% <ø> (-1.58%)	⬇️
#py37	92.07% <ø> (-0.15%)	⬇️
#trial	91.58% <ø> (-1.32%)	⬇️
#windows	94.03% <ø> (-0.01%)	⬇️
#xdist	93.29% <ø> (-0.5%)	⬇️

Impacted Files	Coverage Δ
testing/test_argcomplete.py	18.75% <0%> (-46.88%)	⬇️
src/_pytest/_argcomplete.py	29.41% <0%> (-44.12%)	⬇️
testing/test_pdb.py	52.4% <0%> (-42.52%)	⬇️
src/_pytest/debugging.py	72.86% <0%> (-21.71%)	⬇️
src/_pytest/unittest.py	86.47% <0%> (-7.65%)	⬇️
testing/test_conftest.py	96.72% <0%> (-2.88%)	⬇️
src/_pytest/helpconfig.py	97.19% <0%> (-1.87%)	⬇️
testing/test_terminal.py	99.21% <0%> (-0.79%)	⬇️
src/_pytest/pytester.py	86.58% <0%> (-0.73%)	⬇️

---

Continue to review full report at Codecov.

Legend - Click here to learn more
Δ = absolute <relative> (impact), ø = not affected, ? = missing data
Powered by Codecov. Last update 8bced74...28dbffd. Read the comment docs.

[nicoddemus]

Following what brew recommends in its error message:

	Could not symlink include/c++
	Target /usr/local/include/c++
	already exists. You may want to remove it:
	  rm '/usr/local/include/c++'

@jaraco does this seem correct to you? Any other suggestion?

[nicoddemus]

Oh sorry, I'm mis-remembering, who contributed the macos builds was not @jaraco.

@jirikuncar what do you think?

[blueyed]

Is this symlink in the Travis image already? You might want to ls -l it in the beginning.

[blueyed]

This happens when also upgrading numpy (https://travis-ci.org/pytest-dev/pytest/jobs/444989259#L933), which maybe can be avoided with some option to "brew upgrade python" to not upgrade dependents? (should be faster then, too)

[blueyed]

Pushed a commit that uses brew upgrade --ignore-dependencies python, feel free to revert/amend.

Edit: reverted (d04965b), still failed in the same way (upgrading numpy => gcc).

[nicoddemus]

Thanks for chipping in @blueyed. 👍

Let's see what @jirikuncar has to say.

[nicoddemus]

Well let's get this merged, as having every PR failing because of this is distracting.

[nicoddemus]

(We will need at least 1 approval before we can merge this btw)