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.