Switch to libjpeg-turbo for Windows

[kmilos]

Addresses #140

[kmilos]

Hm, that last commit doesn't really make a difference: for python 3.6, 3.8 and 3.9 numpy (and blas) are grabbed from conda-forge for some reason, while for 3.7 it's happy with the default channel.

conda dependency resolving is just beyond me most of the time...

[letmaik]

Thanks for this. The channel thing is indeed odd. I think -c conda is the default anyway. Can we ping someone from conda to tell us why libjpeg-turbo affects numpy? Seems a bit weird. The PR otherwise is fine anyway. Just think it's a good opportunity to do a little investigation - if it takes too long we'll merge it anyway.

[kmilos]

Can we ping someone from conda to tell us why libjpeg-turbo affects numpy?

No idea who to contact... And I don't think it's libjpeg-turbo per se affecting it, probably just the fact that it sees we're adding the conda-forge channel and decides to grab numpy from there for some version combinations.

[kmilos]

Adding --strict-channel-priority seems to have done the trick.