Update ipykernel requirement from ~=5.5 to >=5.5,<7.0

Updates the requirements on [ipykernel](https://github.com/ipython/ipykernel) to permit the latest version.
- [Release notes](https://github.com/ipython/ipykernel/releases)
- [Changelog](https://github.com/ipython/ipykernel/blob/main/CHANGELOG.md)
- [Commits](ipython/ipykernel@5.5.0...v6.22.0)

---
updated-dependencies:
- dependency-name: ipykernel
  dependency-type: direct:production
...

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

pyproject.toml

@@ -73,7 +73,7 @@ rtd = [
     "altair",
     "bokeh",
     "coconut>=1.4.3,<2.3.0",
-    "ipykernel~=5.5",
+    "ipykernel>=5.5,<7.0",
     "ipywidgets",
     "jupytext>=1.11.2,<1.15.0",
     "matplotlib",
@@ -89,7 +89,7 @@ rtd = [
 testing = [
     "coverage>=6.4,<8.0",
     "beautifulsoup4",
-    "ipykernel~=5.5",
+    "ipykernel>=5.5,<7.0",
     # for issue with 8.1.0 see https://github.com/ipython/ipython/issues/13554
     # TODO ipython 8.5 subtly changes output of test regressions
     # see https://ipython.readthedocs.io/en/stable/whatsnew/version8.html#restore-line-numbers-for-input