CI: Build docker images with Python 3.8

[effigies]

Circle tests are failing because h5py no longer builds wheels for Python 3.6, and our docker images are not equipped to build the wheels. Python 3.6 is on its way out, so upgrading to Python 3.8.

Or should I target 3.9?

[codecov]

Codecov Report

Merging #3287 (ed6ad57) into master (47fe00b) will increase coverage by 0.08%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master    #3287      +/-   ##
==========================================
+ Coverage   64.70%   64.78%   +0.08%     
==========================================
  Files         302      302              
  Lines       39869    39976     +107     
  Branches     5288     5289       +1     
==========================================
+ Hits        25796    25898     +102     
- Misses      12984    12987       +3     
- Partials     1089     1091       +2     

Flag	Coverage Δ
unittests	64.78% <ø> (+0.08%)	⬆️

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

Impacted Files	Coverage Δ
nipype/pipeline/plugins/multiproc.py	76.36% <0.00%> (-2.43%)	⬇️
nipype/pipeline/plugins/linear.py	76.19% <0.00%> (-2.39%)	⬇️
nipype/pipeline/plugins/legacymultiproc.py	63.23% <0.00%> (-2.12%)	⬇️
nipype/pipeline/engine/utils.py	74.22% <0.00%> (-0.26%)	⬇️
nipype/utils/filemanip.py	71.46% <0.00%> (ø)
nipype/interfaces/minc/minc.py	85.18% <0.00%> (+0.01%)	⬆️
nipype/interfaces/fsl/preprocess.py	71.44% <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%)	⬆️
... and 33 more

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[effigies]

In the absence of response, I'm just going to do it.