Bump traitlets from 5.3.0 to 5.4.0 (#1539)

* Bump traitlets from 5.3.0 to 5.4.0

Bumps [traitlets](https://github.com/ipython/traitlets) from 5.3.0 to 5.4.0.
- [Release notes](https://github.com/ipython/traitlets/releases)
- [Commits](ipython/traitlets@5.3.0...5.4.0)

---
updated-dependencies:
- dependency-name: traitlets
  dependency-type: direct:development
  update-type: version-update:semver-minor
...

Signed-off-by: dependabot[bot] <support@github.com>

* Update setup.py

Signed-off-by: dependabot[bot] <support@github.com>
Co-authored-by: dependabot[bot] <49699333+dependabot[bot]@users.noreply.github.com>
Co-authored-by: Mihir Patel <mihir.v.patel7@gmail.com>

setup.py

@@ -125,7 +125,7 @@ def package_files(prefix: str, directory: str, extension: str):
     'sphinx_panels==0.6.0',
     'sphinxcontrib-images==0.9.4',
     'pytest_codeblocks==0.16.1',
-    'traitlets==5.3.0',  # required by testbook. Version 5.2.2 has an import bug, so pinning to 5.1.1, which worked previously.
+    'traitlets==5.4.0',
     'nbsphinx==0.8.9',
     'pandoc==2.2',
     'pypandoc==1.8.1',