Drop python=3.7 support

[adrinjalali]

Now that colab supports 3.8, we should drop 3.7 support.

[BenjaminBossan]

I would be totally happy to drop it. Just wondering what our general policy should be about supported Python versions? For instance, 3.7's end of life is still 6 months away

[adrinjalali]

Ideally we do what everybody else in the pydata stack does, the only reason we haven't done this is that Google Colab is crap.

[BenjaminBossan]

Yes, makes sense, especially since sklearn requires 3.8.