Better docstring detection when folding

[galaunay]

PR Summary

Follow #1774.

In the folding module, we use elpy-info-docstring-p to check if the cursor in on a docstring.
python-info-docstring-p does a way better job at that, and there is no reason (anymore ?) not to use it.

PR checklist

Please make sure that the following things have been addressed (and check the relevant checkboxes):

• Commits respect our guidelines
• Tests are passing properly (see here on how to run Elpy's tests)

[coveralls]

Coverage remained the same at 92.145% when pulling eb46e8e on galaunay:better-docstring-folding into 816061c on jorgenschaefer:master.

[coveralls]

Coverage remained the same at 92.145% when pulling 1cf7788 on galaunay:better-docstring-folding into 816061c on jorgenschaefer:master.

[drdv]

Thanks, this is a nice change and it permits cleaning up other things as well. I would like to suggest another modification related to hiding docstrings. I am not, however, sure what would be the most convenient way for making a PR that depends on this PR.

[gopar]

LGTM

Sorry I haven't active :(