CI: Move from Travis to GitHub actions

[effigies]

Summary

Fixes #3274.

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 #3318 (17a9827) into master (9e8b655) will increase coverage by 0.30%.
The diff coverage is n/a.

❗ Current head 17a9827 differs from pull request most recent head 615887c. Consider uploading reports for the commit 615887c to get more accurate results

@@            Coverage Diff             @@
##           master    #3318      +/-   ##
==========================================
+ Coverage   64.70%   65.00%   +0.30%     
==========================================
  Files         302      302              
  Lines       39911    40027     +116     
  Branches     5291     5292       +1     
==========================================
+ Hits        25825    26021     +196     
+ Misses      12998    12934      -64     
+ Partials     1088     1072      -16     

Flag	Coverage Δ
unittests	64.70% <ø> (ø)

Flags with carried forward coverage won't be shown. Click here to find out more.

Impacted Files	Coverage Δ
nipype/algorithms/icc.py	57.53% <0.00%> (ø)
nipype/utils/docparse.py	52.21% <0.00%> (ø)
nipype/interfaces/fsl/utils.py	63.76% <0.00%> (ø)
nipype/interfaces/minc/minc.py	85.18% <0.00%> (+0.01%)	⬆️
nipype/interfaces/fsl/preprocess.py	71.54% <0.00%> (+0.03%)	⬆️
nipype/interfaces/ants/registration.py	73.09% <0.00%> (+0.05%)	⬆️
nipype/interfaces/base/specs.py	89.28% <0.00%> (+0.05%)	⬆️
nipype/interfaces/base/traits_extension.py	88.29% <0.00%> (+0.06%)	⬆️
nipype/testing/fixtures.py	98.70% <0.00%> (+0.09%)	⬆️
nipype/interfaces/niftyreg/regutils.py	80.76% <0.00%> (+0.09%)	⬆️
... and 37 more

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[effigies]

Re-adding style and specs checks will need to be separate. black.format_str isn't generating text that black is happy with, so I'm pushing that off for now.

@djarecka @satra @oesteban @mgxd (I'm not sure who all considers themselves an active maintainer rn) does this cover the use cases that Travis used to enough to be going on with?

[satra]

looks reasonable