Revert "ci: Travis: add pypy3 to allowed failures temporarily"

[blueyed]

This reverts commit 5ac498e.

The idea is that maybe #5360
fixes the failures here also.

[codecov]

Codecov Report

Merging #5364 into master will decrease coverage by 2.17%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master    #5364      +/-   ##
==========================================
- Coverage   93.43%   91.26%   -2.18%     
==========================================
  Files         115      115              
  Lines       26399    26399              
  Branches     2606     2606              
==========================================
- Hits        24667    24094     -573     
- Misses       1409     1981     +572     
- Partials      323      324       +1

Impacted Files	Coverage Δ
testing/test_reports.py	10.49% <0%> (-89.51%)	⬇️
testing/test_session.py	17.72% <0%> (-82.28%)	⬇️
testing/test_runner_xunit.py	29.82% <0%> (-70.18%)	⬇️
testing/test_runner.py	34.7% <0%> (-61.61%)	⬇️
testing/test_resultlog.py	53.6% <0%> (-46.4%)	⬇️
testing/test_pytester.py	55.21% <0%> (-34.09%)	⬇️
src/_pytest/reports.py	68.96% <0%> (-26.11%)	⬇️
testing/test_tmpdir.py	75.8% <0%> (-23.12%)	⬇️
testing/test_skipping.py	72.77% <0%> (-23.06%)	⬇️
src/_pytest/resultlog.py	67.6% <0%> (-19.72%)	⬇️
... and 21 more

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