DOC: Skip BIDSDataGrabber doctest if pybids is missing

[effigies]

Summary

Fixes #3223.

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]

Codecov Report

Merging #3224 into maint/1.5.x will increase coverage by 0.04%.
The diff coverage is 100.00%.

@@               Coverage Diff               @@
##           maint/1.5.x    #3224      +/-   ##
===============================================
+ Coverage        64.96%   65.01%   +0.04%     
===============================================
  Files              302      302              
  Lines            39929    39930       +1     
  Branches          5279     5279              
===============================================
+ Hits             25939    25959      +20     
+ Misses           12925    12913      -12     
+ Partials          1065     1058       -7     

Flag	Coverage Δ
#unittests	65.01% <100.00%> (+0.04%)	⬆️

Impacted Files	Coverage Δ
nipype/interfaces/io.py	56.41% <ø> (ø)
nipype/conftest.py	96.15% <100.00%> (+0.15%)	⬆️
nipype/pipeline/plugins/base.py	60.00% <0.00%> (+1.91%)	⬆️
nipype/pipeline/plugins/legacymultiproc.py	71.56% <0.00%> (+5.88%)	⬆️

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[effigies]

Should be pretty uncontroversial.