Misc typing fixes (#6021)

* remove unnecessary annotations

* os.path.relpath accepts a Path

* simpler handling and typechecking of cachecontrol

* remove unneeded type assertion

* os.path.exists accepts a Path

* tighten the handling of env on commands

* tighten handling of installer on commands

* tighten handling of environment in poetry shell

* tighten typechecking in authenticator