Update wcsaxes and pvextractor

While working on the PV extractor, I came across some issues in how the clip path is handled in WCSAxes (https://github.com/astrofrog/wcsaxes/pull/117 and https://github.com/astrofrog/wcsaxes/pull/120). I also realized that some of the bugs that are apparent in WCSAxes in glue have already been fixed, so this PR updates WCSAxes and the pvextractor package to the latest developer versions.

@ChrisBeaumont - please try this out to check that it still works well for you!