use extras_require to specify tests

[bauer-at-work]

Issue #, if available:

Description of changes:

Specify test dependency in extras_require instead of tests_require.
Reason: The latter is deprecated. See https://github.com/pypa/setuptools/blob/bb0506352435a13f6fd70fa3b9eae6fd0f6e1ea8/docs/references/keywords.rst?plain=1#L452-L467

By submitting this pull request, I confirm that my contribution is made under the terms of the Apache 2.0 license.

P.S.: This change (together with setting environment variable NODE_OPTIONS="--openssl-legacy-provider" for a totally different reason), allows to specify graph-notebook as a git-based dependency in a pyproject.toml and to install it using poetry:

[tool.poetry.group.dev.dependencies]
graph-notebook = { git = "https://github.com/bauer-at-work/graph-notebook", branch = "main" }

[michaelnchin]

Thanks @bauer-at-work !