CI: Update CircleCI machine image

[effigies]

Builds are failing because coverage uses f-strings in its setup.py, and we're working on Python 3.6 images. It's not worth pinning an old coverage with Py36 EOL at the end of the year.

[codecov]

Codecov Report

Merging #3391 (cbc714f) into master (d8ba317) will decrease coverage by 0.85%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master    #3391      +/-   ##
==========================================
- Coverage   65.13%   64.27%   -0.86%     
==========================================
  Files         307      307              
  Lines       40372    40372              
  Branches     5336     5336              
==========================================
- Hits        26297    25950     -347     
- Misses      13003    13368     +365     
+ Partials     1072     1054      -18     

Flag	Coverage Δ
unittests	?

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Impacted Files	Coverage Δ
nipype/workflows/__init__.py	47.05% <0.00%> (-52.95%)	⬇️
nipype/utils/spm_docs.py	20.00% <0.00%> (-48.00%)	⬇️
nipype/interfaces/freesurfer/base.py	48.38% <0.00%> (-28.23%)	⬇️
nipype/utils/logger.py	56.92% <0.00%> (-27.70%)	⬇️
nipype/testing/fixtures.py	77.92% <0.00%> (-20.78%)	⬇️
nipype/interfaces/dcm2nii.py	50.23% <0.00%> (-14.89%)	⬇️
nipype/interfaces/ants/base.py	65.30% <0.00%> (-8.17%)	⬇️
nipype/interfaces/spm/base.py	60.06% <0.00%> (-7.60%)	⬇️
nipype/interfaces/io.py	48.09% <0.00%> (-7.15%)	⬇️
nipype/interfaces/cat12/surface.py	59.34% <0.00%> (-6.60%)	⬇️
... and 11 more

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