Styling: standard library should be imported before custom library

[adriantam]

Currently, the client/client.py imports custom libraries before the standard libraries as per PEP 8 style. The order should be

1. Standard library imports.
2. Related third party imports.
3. Local application/library specific imports.

Furthermore, it may be good to have clean up the import so that we only import what is needed (#23).

Ideally, we will also fix the SDK generator so that newly generated python SDKs will have the corresponding changes. The corresponding SDK generator issue is openfga/sdk-generator#126. However, if that is difficult to do, simply focusing on the Python SDK side https://github.com/openfga/python-sdk/blob/main/openfga_sdk/client/client.py will help us as well.