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

As per the RFC
https://discourse.llvm.org/t/rfc-upgrading-llvms-minimum-required-python-version/67571,
raise the minimum Python version to ensure that the Python syntax
doesn't become overly obsolete, to enable new Python feature usage,
and to improve the maintainability of CI.

One of the primary use cases for this higher Python version is to enable
python type annotations that are more aligned with current Python
best practices. This is not only important for our own internal Python
for testing, but for the Python bindings that are exposed to users.