CI: removing travis_retry for pip install pytest xdist 1.22.5

[djarecka]

reverse #2659
It looks like the new release of xdist resolve the dependencies better and travis retry not needed.

List of changes proposed in this PR (pull-request)

Acknowledgment

• (Mandatory) I acknowledge that this contribution will be available under the Apache 2 license.

[codecov-io]

Codecov Report

Merging #2664 into master will decrease coverage by 3.48%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master    #2664      +/-   ##
==========================================
- Coverage   67.59%   64.11%   -3.49%     
==========================================
  Files         340      338       -2     
  Lines       43054    43003      -51     
  Branches     5329     5326       -3     
==========================================
- Hits        29104    27570    -1534     
- Misses      13248    14375    +1127     
- Partials      702     1058     +356

Flag	Coverage Δ
#smoketests	?
#unittests	64.11% <ø> (-0.94%)	⬇️

Impacted Files	Coverage Δ
nipype/interfaces/nilearn.py	40% <0%> (-56.67%)	⬇️
nipype/utils/spm_docs.py	25.92% <0%> (-44.45%)	⬇️
nipype/interfaces/nitime/analysis.py	43.15% <0%> (-36.85%)	⬇️
nipype/interfaces/freesurfer/base.py	49.59% <0%> (-30.9%)	⬇️
nipype/utils/logger.py	59.7% <0%> (-29.86%)	⬇️
nipype/algorithms/rapidart.py	35.39% <0%> (-29.21%)	⬇️
nipype/interfaces/spm/base.py	58.41% <0%> (-29.05%)	⬇️
nipype/utils/provenance.py	55.73% <0%> (-28.99%)	⬇️
nipype/interfaces/fsl/model.py	55.26% <0%> (-25.35%)	⬇️
nipype/testing/fixtures.py	77.33% <0%> (-21.34%)	⬇️
... and 45 more

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