[Python] Correct README documentation and client.py import order

[adriantam]

Need to update the python client generator for the following bug

• Styling: README should import only the class and method instead of entire module python-sdk#23
• Styling: standard library should be imported before custom library python-sdk#24
• README client should call fga_client instead of api_instance python-sdk#25
• README: use different code block for different read API configuration python-sdk#26
• README: double slash used as comments in example python-sdk#27
• README: add import and client initialization as comments for each code block python-sdk#28
• README: break up WriteAuthorizationModelRequest into smaller understandable pieces python-sdk#29