Travis: run pytest --help for (stable) TTY coverage

[blueyed]

According to https://codecov.io/gh/pytest-dev/pytest/pull/4881/changes
the "isatty" branches in _pytest/terminal.py are flaky.

This tries to fix it.

Maybe running it in a subprocess would also help, but it might be more about the outer term in the first place.

Travis should behave as a TTY AFAIK.

[codecov]

Codecov Report

Merging #4884 into master will increase coverage by 0.06%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master    #4884      +/-   ##
==========================================
+ Coverage   94.47%   94.53%   +0.06%     
==========================================
  Files         113      113              
  Lines       25173    25173              
  Branches     2499     2499              
==========================================
+ Hits        23782    23798      +16     
+ Misses       1060     1048      -12     
+ Partials      331      327       -4

Impacted Files	Coverage Δ
src/_pytest/terminal.py	91.74% <0%> (+2.64%)	⬆️

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[blueyed]

Also increased/covered without.