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

[Aiden Grossman via llvm-commits]

boomanaiden154 wrote:

Actually seems to be here: https://github.com/llvm/llvm-project/blob/53fea6fd6f228eb1dbd282b8d076af545dcd8228/.ci/monolithic-windows.sh#L40C64-L41C1

It seems like the configuration and the installation are using the same Python interpreter:
```
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.
```

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