Travis: use pypy from Xenial, add pypy3

[blueyed]

No description provided.

[asottile]

[codecov]

Codecov Report

Merging #4757 into master will increase coverage by <.01%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master    #4757      +/-   ##
==========================================
+ Coverage   95.48%   95.49%   +<.01%     
==========================================
  Files         113      113              
  Lines       25150    25041     -109     
  Branches     2498     2486      -12     
==========================================
- Hits        24015    23913     -102     
+ Misses        803      798       -5     
+ Partials      332      330       -2

Flag	Coverage Δ
#docs	29.55% <ø> (+0.04%)	⬆️
#doctesting	29.55% <ø> (+0.04%)	⬆️
#linting	29.55% <ø> (+0.04%)	⬆️
#linux	95.49% <ø> (ø)	⬆️
#nobyte	91.58% <ø> (ø)	⬆️
#numpy	42.09% <ø> (+0.11%)	⬆️
#pexpect	42.09% <ø> (+0.11%)	⬆️
#pluggymaster	93.56% <ø> (+0.01%)	⬆️
#py27	93.56% <ø> (-0.01%)	⬇️
#py34	91.56% <ø> (+0.08%)	⬆️
#py35	91.56% <ø> (+0.06%)	⬆️
#py36	91.54% <ø> (+0.05%)	⬆️
#py37	93.63% <ø> (-0.02%)	⬇️
#trial	42.09% <ø> (+0.11%)	⬆️
#xdist	93.47% <ø> (-0.07%)	⬇️

Impacted Files	Coverage Δ
src/_pytest/warning_types.py	92.85% <0%> (-7.15%)	⬇️
src/_pytest/cacheprovider.py	95.75% <0%> (-1.42%)	⬇️
testing/test_session.py	96.37% <0%> (-0.32%)	⬇️
testing/test_mark.py	99.06% <0%> (-0.32%)	⬇️
testing/test_config.py	99.42% <0%> (-0.21%)	⬇️
testing/test_pytester.py	87.45% <0%> (-0.13%)	⬇️
src/_pytest/config/__init__.py	93.67% <0%> (-0.13%)	⬇️
src/_pytest/main.py	95.93% <0%> (-0.09%)	⬇️
testing/test_assertrewrite.py	83.48% <0%> (-0.08%)	⬇️
src/_pytest/helpconfig.py	97.24% <0%> (-0.05%)	⬇️
... and 15 more

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[nicoddemus]

Not waiting on AppVeyor as it does not apply there.