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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