refactor: remove Python 2 compatibility

[evansims]

[!IMPORTANT]
Disregard any failing tests on this pull request; this is due to changes in our code styling tools. Once this PR is merged, future PRs will pass correctly.

[!IMPORTANT]
Failing tests around urllib3 should be ignored for the moment. These will be addressed separately in future PRs.

This PR:

• Removes legacy Python 2 code, as that release is unsupported.
• Removes dependencies that were required for Python 2 support.
• Updates the SDK to use newer language features available in Python 3.
• Updates the code styling and quality tools.

Description

This PR makes several improvements to the Python SDK:

• Removes the six dependency, which was only present for Python 2 compatibility. Imports and references to the library have been removed from the SDK codebase.
• Improves the requirements.txt and test-requirements.txt files by imposing stricter version ranges to avoid potential future conflicts when new majors of dependencies are released.
• Improves the setup.py file by removing the redundant dependency version manifest. It now imports the existing requirements.txt file, as the manifests are identical.
• Cleans up imports, removing unused references and logically sorting lists.
• Removes unnecessary inline code formatting rules. (# noqa: ..., etc.)
• Removes the # coding: utf-8 header line from source files, as this is the default assumed in Python 3.
• Adopts Python 3 language features and best practices.

Besides this, it introduces a number of code formatting tools to produce a cleaner, more efficient SDK result:

• black is an opinionated code formatter.
• isort automatically sorts imports, and efficiently groups them when appropriate.
• autoflake removes unused imports, which is particularly useful here given the conditional usage of certain imports in our templates, such as modules.
• pyupgrade identifies and optimizes code for Python 3.10+ where appropriate.

References

Closes #77

Review Checklist

• [x] I have clicked on "allow edits by maintainers".
• [ ] I have added documentation for new/changed functionality in this PR or in a PR to openfga.dev [Provide a link to any relevant PRs in the references section above]
• [x] The correct base branch is being used, if not main
• [ ] I have added tests to validate that the change in functionality is working as expected