[Python] Drop support for python 3.8

[msyyc]

Since Python 3.8 is end of life, python emitter shall drop support for it.

• microsoft/typespec repo: [python] drop python 3.8  #6821
• autorest.python repo: drop python 3.8 Azure/autorest.python#3065
• SDK repo: drop python 3.8 for codegen and bump typespec-python / autorest.python  Azure/azure-sdk-for-python#40396