Update ipython requirement from ==8.12.* to ==8.21.*

Updates the requirements on [ipython](https://github.com/ipython/ipython) to permit the latest version.
- [Release notes](https://github.com/ipython/ipython/releases)
- [Commits](ipython/ipython@8.12.0...8.21.0)

---
updated-dependencies:
- dependency-name: ipython
  dependency-type: direct:development
...

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

requirements-dev.txt

@@ -4,7 +4,7 @@ codespell
 ddtrace
 # Pin IPython to 8.12 when building on Python 3.8
 # https://github.com/ipython/ipython/blob/main/README.rst
-ipython == 8.12.*; python_version == '3.8'
+ipython == 8.21.*; python_version == '3.8'
 ipython; python_version > '3.8'
 jinja2
 mkdocs