Bump ipywidgets from 7.6.4 to 7.7.0 in /extra_requirements (#348)

Bumps [ipywidgets](http://ipython.org) from 7.6.4 to 7.7.0.

---
updated-dependencies:
- dependency-name: ipywidgets
  dependency-type: direct:production
  update-type: version-update:semver-minor
...

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

Co-authored-by: dependabot[bot] <49699333+dependabot[bot]@users.noreply.github.com>
Co-authored-by: Joran R. Angevaare <jorana@nikhef.nl>

extra_requirements/requirements-tests.txt

@@ -15,7 +15,7 @@ coverage==6.3.2
 epix==0.2.2
 flake8==4.0.1
 holoviews==1.14.5
-ipywidgets==7.6.4
+ipywidgets==7.7.0
 hypothesis==6.39.0
 jinja2==3.0.3                                  # Holoviews dependency
 jupyter-client==7.1.2                          # for ipywidgets