Add support for python 3.8

[ssteinbach]

It seems as though it is time to add in python 3.8 support. VFX Platform skips to 3.7 so we should be good to swap 3.6 for 3.8.

[apetrynet]

Other than a lot of versions to support, is there a reason not to keep python 3.6 support?
I think centos7 stops at 3.6 without adding additional repositories or compiling.

[ssteinbach]

We're just dropping the entry from our CI -- trying to keep our support load closely tied to the VFX platform.

[apetrynet]

OK

[ssteinbach]

Closed by #787