MNT: Require Python 3.8+, auto-upgrade syntax

[effigies]

Drops Python 3.7 support, and ran pyupgrade --py38-plus a couple times along with black and some manual tweaks.

[codecov]

Codecov Report

Patch coverage: 51.18% and project coverage change: -0.09 ⚠️

Comparison is base (7d4808f) 63.28% compared to head (7958828) 63.20%.

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #3588      +/-   ##
==========================================
- Coverage   63.28%   63.20%   -0.09%     
==========================================
  Files         308      308              
  Lines       40876    40787      -89     
  Branches     5485     5484       -1     
==========================================
- Hits        25870    25779      -91     
- Misses      13995    13997       +2     
  Partials     1011     1011              

Impacted Files	Coverage Δ
nipype/algorithms/__init__.py	100.00% <ø> (ø)
nipype/algorithms/icc.py	58.97% <ø> (ø)
nipype/algorithms/mesh.py	29.70% <0.00%> (ø)
nipype/algorithms/metrics.py	56.28% <ø> (ø)
nipype/algorithms/stats.py	100.00% <ø> (ø)
nipype/caching/__init__.py	100.00% <ø> (ø)
nipype/interfaces/__init__.py	100.00% <ø> (ø)
nipype/interfaces/afni/__init__.py	100.00% <ø> (ø)
nipype/interfaces/afni/svm.py	97.61% <0.00%> (ø)
nipype/interfaces/ants/__init__.py	100.00% <ø> (ø)
... and 207 more

... and 68 files with indirect coverage changes

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