Revert "Bump min python to 3.9"

Since python 3.9 adds support for using list and dict as types this has made
ruff unhappy with using the typing constructs of List and Dict. Reverting my
change for now; we can revisit in separate PR.

This reverts commit d0d31d3.

Author: sjmonson

pyproject.toml

@@ -20,7 +20,7 @@ name = "guidellm"
 version = "0.1.0"
 description = "Guidance platform for deploying and managing large language models."
 readme = { file = "README.md", content-type = "text/markdown" }
-requires-python = ">=3.9.0,<4.0"
+requires-python = ">=3.8.0,<4.0"
 license = { file = "LICENSE" }
 authors = [ { name = "Neuralmagic, Inc." } ]
 urls = { homepage = "https://github.com/neuralmagic/guidellm" }