Drop support for Python 3.8

[Pete Gadomski]

We've had a de-facto policy to follow numpy's deprecation policy, https://numpy.org/neps/nep-0029-deprecation_policy.html#support-table, where Python 3.8 support was dropped on 2023-04-14.

We should probably also update our docs to explicitly state that we're following numpy's deprecation policy.

Once this issue is closed, the same should be done for:

• pystac-client
• stactools

[Julia Signell]

🔥 🔥 maybe we can run pyupgrade on the code while we are at it to make sure we are taking advantage of the latest and greatest features.

[Julia Signell]

Just a note that there is an importlib_resources backport that we can drop once we drop python 3.8

[Pete Gadomski]

We should probably also update our docs to explicitly state that we're following numpy's deprecation policy.

Whoops, we already do this:

pystac/docs/installation.rst

Lines 30 to 34 in 9c323c4

 PySTAC requires Python >= 3.8. This project follows the recommendations of 
 `NEP-29 <https://numpy.org/neps/nep-0029-deprecation_policy.html>`__ in deprecating support 
 for Python versions. This means that users can expect support for Python 3.8 to be 
 removed from the ``main`` branch after Apr 14, 2023 and therefore from the next release 
 after that date. 

🔥 🔥 maybe we can run pyupgrade on the code while we are at it to make sure we are taking advantage of the latest and greatest features.

I've opened a new issue to track this (#1235), since I want to separate the "remove 3.8" PR from the "run pyupgrade" PR (since that'll be noisy).