Adopt pooch

[bjlittle]

Would we be willing to adopt pooch as a more robust way to download remote assets?

If there's enough interest for this, then I'm happy to investigate further and see whether pooch can easily be integrated into the existing framework.

[bjlittle]

The main advantages, as I see them, from adopting pooch are:

• timeout and retry management through pooch
• validation of download payload (through payload sha's)
• compression/decompression support of assets (although probably not much gain from compressing pngs)
• pooch won't re-download an existing assets i.e., provides cache support
• we drop our bespoke code to do this

There may be more ... but I'd be interested to know whether you already know this isn't a good fit for pytest-mpl and has been rejected as an option, or whether there's a blocker/disadvantage that I'm not aware of ...