Remove support for Python 3.8

[zanieb]

I'll fill in some details here, but we should probably simply the builds by dropping support for 3.8 now that it is EOL. I'm not sure how long we want to support versions after they're EOL, but the build matrix is huge as-is.

[indygreg]

I've historically dropped support (read: stopped publishing and deleted code to build) Python versions once they hit EOL per upstream.

So it is time to remove 3.8 from PBS.