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.