remove support for Python 3.8

[philvarner]

Related Issue(s):

• n/a

Proposed Changes:

1. Python 3.8 is EOL this year, so there are no new projects that should be using it, and existing projects don't need to update to the next major-minor.
2. In the last two changes I've tried to make, I've had incompatibilities with 3.8 that have required a workaround, so supporting it is becoming a burden.

PR Checklist:

• I have added my changes to the CHANGELOG or a CHANGELOG entry is not required.

[gadomski]

👍🏼 we dropped it from pystac, where we have the policy of following numpy.

Could add 3.12?

[philvarner]

Could add 3.12?
It's already in there.

[gadomski]

It's already in there.

🤦🏼

[philvarner]

Also @gadomski, the only reason I was reluctant to remove it was we have this idea that it may be possible to convert an existing container that runs processing (possibly on an older version of Python b/c scientists) into a stactask Task, but I think that's not feasible in most cases, so need to find another way to do that anyway.