Handle Sphinx 1.3 API change

[blink1073]

Sphinx changed cfg.source_suffix to be a list in version 1.3.
We were using it as a string.
First, I tried just changing cfg.source_suffix to a string if it was a list, but that affected other parts of the build. Instead, we create a new cfg.source_suffix_str and use that, so we can support all versions of Sphinx.

[blink1073]

@jni, you may just want to take these two lines and add them to #1416.

[jni]

@blink1073 when and if this passes and gets merged (=P), I'll rebase... I like to keep PRs small and self-contained...

[blink1073]

Why sphinx why?!?!?

[tacaswell]

mpl is also having issues. We have pinned to 1.2.3 for now...

cc @jenshnielsen

[coveralls]

Coverage remained the same at 92.7% when pulling 7d65366 on blink1073:contrast-check-function into d4463ba on scikit-image:master.

[blink1073]

@scikit-image/core, someone please merge this as it is holding up all open PRs.

[coveralls]

Coverage remained the same at 92.7% when pulling 5c6ee95 on blink1073:contrast-check-function into d4463ba on scikit-image:master.

[JDWarner]

Travis is happy, AppVeyor is still queued 4 hours later. I realize it's a free service and resources are limited, but that kind of delay is borderline unusable.

Thanks for the fix @blink1073 - I'm merging this. If Windows ends up having a problem we can deal with that separately.