[llvm] [python] Bump Python minimum version to 3.8 (PR #78828)

[Craig Hesling via llvm-commits]

linux4life798 wrote:

Very interesting. It looks like the same buildkite Windows x64 job now succeeds. For referencing later, it is [buildkite build #37898](https://buildkite.com/llvm-project/github-pull-requests/builds/37898). I'm going to rebase and push again to see if this buildkite build is flaky.

---

> ```
> pip install -q -r C:/ws/src/mlir/python/requirements.txt
> WARNING: You are using pip version 21.2.3; however, version 23.2.1 is available.
> You should consider upgrading via the 'C:\Python39\python.exe -m pip install --upgrade pip' command.
> ```

Hmm. I'm not quite sure how the pip upgrade warning would create issues. That's usually a harmless reminder, but I could be wrong.


https://github.com/llvm/llvm-project/pull/78828