Pandas 0.24 compatibility and CI fixes

[philippjfr]

The pandas dtype extensions changed slightly in 0.24 breaking HoloViews in major ways in the process. Apart from updating the import, to avoid this in future I've split the try/except blocks to separate the extension types and added a warning.

[jbednar]

Thanks. Are there any compatibility patches you could propose for Pandas itself, to avoid such problems with other libraries?

[philippjfr]

Are there any compatibility patches you could propose for Pandas itself, to avoid such problems with other libraries?

No, this is entirely on us (or rather me).

[philippjfr]

I'm going to merge this shortly. Basically there are two remaining issues:

• Gif and mp4 renderer tests are basically skipped in linux right now, I tried fixing this even going so far as trying to reproduce it in a linux virtual machine but had no luck.
• Appveyor builds are currently broken (across all our projects), I've allowed them to go green despite this failure for now and filed an issue here: Miniconda permission errors appveyor/ci#2822

The PR does the following:

• Update to all latest versions (including pandas 0.24.0, matplotlib, xarray etc.)
• Compatibility for pandas 0.24.0
• Add appveyor.yml (which was briefly passing until the miniconda on appveyor was updated and broken)

[github-actions]

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