MNT: Set junit_family to suppress pytest warning

[effigies]

Current tests emit this warning from pytest:

/opt/miniconda-latest/envs/neuro/lib/python3.6/site-packages/_pytest/junitxml.py:436

  /opt/miniconda-latest/envs/neuro/lib/python3.6/site-packages/_pytest/junitxml.py:436: PytestDeprecationWarning: The 'junit_family' default value will change to 'xunit2' in pytest 6.0.

  Add 'junit_family=legacy' to your pytest.ini file to silence this warning and make your suite compatible.

    _issue_warning_captured(deprecated.JUNIT_XML_DEFAULT_FAMILY, config.hook, 2)

In niworkflows, we were able to suppress it by making the included change. Selected the planned default value to embrace the future. If it fails, we can cling to the past.

[effigies]

Tests pass and warnings don't appear in https://circleci.com/gh/nipy/nipype/10593.

Will merge on green checks unless someone objects.

[codecov]

Codecov Report

Merging #3111 into master will decrease coverage by 0.03%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master    #3111      +/-   ##
==========================================
- Coverage   67.86%   67.82%   -0.04%     
==========================================
  Files         295      295              
  Lines       39263    39369     +106     
  Branches     5172     5204      +32     
==========================================
+ Hits        26646    26703      +57     
- Misses      11912    11955      +43     
- Partials      705      711       +6

Flag	Coverage Δ
#smoketests	51.23% <ø> (ø)	⬆️
#unittests	65.06% <ø> (-0.04%)	⬇️

Impacted Files	Coverage Δ
nipype/interfaces/dcm2nii.py	58.68% <0%> (-6.11%)	⬇️
nipype/interfaces/spm/model.py	47.11% <0%> (+4.84%)	⬆️

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