Support for xunit2 junit style with filenames

Newer versions of pytest use xunit2 instead of xunit1 format, described here:

One consequence is that xunit2 no longer supports the file attribute on the <testcase> element, only classname. Notably, the example in Circle CI docs no longer works on the newest pytest (Running Tests in Parallel - CircleCI), namely because circleci tests glob returns filenames, but the junit file contains class names.

As a workaround, we’re doing

circleci tests glob "python/test/**/*.py" \
  | sed -e 's;/;.;g' -e 's/\.py$//' \
  | circleci tests split --split-by=timings \
  | sed -e 's;\.;/;g' -e 's/$/.py/'

But ideally, it would be nice if circleci tests split could automatically coerce filepaths into classnames (or do so with a flag), considering filepaths are no longer allowed on <testcase> elements.

So Circle CI needs to either update the documentation to work (e.g. with the workaround specified here) or fix circleci tests split as mentioned above.