MNT: Execute slowest tests on schedule

[jhlegarreta]

Execute slowest tests on schedule.

Fixes #229.

[jhlegarreta]

@effigies I am not happy with this duplication, but did not find a better way to try to do this:
https://github.com/nipy/nitransforms/pull/258/files#diff-8a1944567ec5d9e12aaaac28489c47e6567e13b58d861815379f9b8d3aeb2623R115-R159

[oesteban]

I dropped 3.9 in #259 because I believe version 25.x series should drop it (and backward compatibility of #256 was really hard).

That all said, what takes the most time here is CircleCI's tests. Perhaps we can drop all the docker shebang which substantially adds to the build.

[jhlegarreta]

That all said, what takes the most time here is CircleCI's tests. Perhaps we can drop all the docker shebang which substantially adds to the build.

Please, feel free to take over; I thought that issue #229 referred to Python testing purely, so did not have a look at what are other bottlenecks. I am not sure the current approach is working well either, since I do not see the GitHub checks taking place on the commit event.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 96.74%. Comparing base (0c2ad9f) to head (34e33e7).

Additional details and impacted files

@@           Coverage Diff           @@
##           master     #258   +/-   ##
=======================================
  Coverage   96.74%   96.74%           
=======================================
  Files          16       16           
  Lines        1994     1994           
  Branches      267      267           
=======================================
  Hits         1929     1929           
  Misses         41       41           
  Partials       24       24           

Flag	Coverage Δ
unittests	96.72% <ø> (ø)

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