AppVeyor: Workaround for PermissionError during shutil.rmtree

[mbargull]

Checklist

• Used a fork of the feedstock to propose changes
• Bumped the build number (if the version is unchanged)
• Reset the build number to 0 (if the version changed)
• Re-rendered with the latest conda-smithy
• Ensured the license file is being packaged.

Might resolve gh-14 or at least make it occur less frequently.

It looks like it's very possible that the cleanup shutil.rmtree(dn) call fails on Windows due to open file handles or such. Sleeping a short while and trying again might help. Seems like conda-build suffers the same fate, see conda-forge/conda-feedstock#52.
NOTE: That 0.1 second duration is chosen arbitrarily; maybe it could be shorter, maybe it should be longer, I don't know.

[conda-forge-linter]

Hi! This is the friendly automated conda-forge-linting service.

I just wanted to let you know that I linted all conda-recipes in your PR (recipe) and found it was in an excellent condition.

[mbargull]

If you think this change makes sense, should this get a build or a version number bump? I assume the latter?

[scopatz]

I believe that @CJ-Wright has seen this in a few cases and has a more robust solution for windows, somewhere. Where was that lying?

[mbargull]

and has a more robust solution for windows, somewhere

That'd be great! I know @kalefranz has been (and I think still is?) battling those beasts for conda, too. (I deliberately ignored those issues and his progress on this, though -- sorry Kale 😉.)
Be sure to let us know if you got something on this, @CJ-Wright, please 😄.

[CJ-Wright]

I think you are looking for https://github.com/regro/rever/blob/master/tests/conftest.py#L44

[scopatz]

Yup that is it!