[python] remove logic related with python 2

[msyyc]

There is still some code (e.g.

typespec/packages/http-client-python/generator/pygen/codegen/models/imports.py

Line 27 in 992b2c3

CONDITIONAL = "conditional" # is a typing import when we're dealing with files that py2 will use, else regular

) related with python 2 which is not supported anymore.

Task:
Remove code related with python 2 for folder https://github.com/microsoft/typespec/tree/main/packages/http-client-python/generator/pygen