Add isort configuration to pyproject.toml

[grlee77]

Description

This PR adds an isort configuration to pyproject.toml. To avoid introducing conflicts, I have not run isort with this configuration on the code base. It can be used by devs to manually update specific, individual files in new PRs (e.g. isort <filename>). Without this configuration, it is hard to remember the command line options and apply them consistently.

In the future, we could potentially configure something with pre-commit to automatically apply this to any modified files.

Checklist

• Docstrings for all functions
• Gallery example in ./doc/examples (new features only)
• Benchmark in ./benchmarks, if your changes aren't covered by an
  existing benchmark
• Unit tests
• Clean style in the spirit of PEP8

For reviewers

• Check that the PR title is short, concise, and will make sense 1 year
  later.
• Check that new functions are imported in corresponding __init__.py.
• Check that new features, API changes, and deprecations are mentioned in
  doc/release/release_dev.rst.