Dev install documentation and dependencies fix

[ian-fox]

• Update contributing.md to reflect the change from poetry to setuptools
• Add attrs as a dependency in pyproject.toml

Attrs is imported in structures.py but is not actually installed by default. It appears to be some transitive dependency of one of the dev dependencies, so it works fine if you install in dev mode but if you're installing the package without that you got an import error.

I thought this was small enough that it wasn't worth adding a test in the suite for, but if you disagree please let me know and I can add one.

Checklist

• [ ] If this PR is a bug fix, the bug is documented in the test suite.
• [x] Changes were documented in the changelog (pending section).
• [x] If necessary, changes were made to the documentation.

[codecov[bot]]

Codecov Report

Patch and project coverage have no change.

Comparison is base (42473ae) 94.78% compared to head (e724e13) 94.78%.

Additional details and impacted files

```diff @@ Coverage Diff @@ ## main #16 +/- ## ======================================= Coverage 94.78% 94.78% ======================================= Files 32 32 Lines 1974 1974 ======================================= Hits 1871 1871 Misses 103 103 ```

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[percevalw]

Hi ! thanks for this, could you rebase against the latest commit of main? That should fix the documentation check

[ian-fox]

Thanks for the feedback, applied both and rebased!