Fix black exclude parameter in pyproject.toml

[emilykl]

@gvwilson observed an issue where running python commands.py codegen on a fresh pull from main shows a LARGE number of changed files via git status. Furthermore the new files seem to not have black formatting applied (they are full of single-quoted strings, etc.).

This issue turns out to be caused by the [tool.black] exclude parameter in pyproject.toml. The regular expression was missing a leading and trailing / character, which caused it to match every file path containing one of the listed strings, rather than just including directories with those names.

Since the directory graph_objs contains the string js, this caused a significant number of files to be excluded from black formatting. Some files were also excluded because their path contained dist.

This PR adds leading and trailing / characters to the regex so that only directories of those names are excluded, as in the example here. (yes, not Windows-friendly but there are already `/'s in the exclude).

Ultimately we need to clean all of this up and switch to ruff but this is a simple fix in the meantime.

How to test

1. Pull this branch
2. Run python commands.py codegen
3. Run git status. You should see zero unstaged changes.

You can also run these steps on main to compare; running python commands.py codegen on main results in a large number of unstaged changes.

[gvwilson]

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