Update ipympl requirement from ~=0.9.3 to ~=0.9.4

Updates the requirements on [ipympl](https://github.com/matplotlib/ipympl) to permit the latest version.
- [Release notes](https://github.com/matplotlib/ipympl/releases)
- [Changelog](https://github.com/matplotlib/ipympl/blob/main/RELEASE.md)
- [Commits](matplotlib/ipympl@0.9.3...0.9.4)

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

Signed-off-by: dependabot[bot] <[email protected]>

requirements.txt

@@ -11,6 +11,6 @@ seekpath~=2.1.0
 plotly~=5.22.0
 nglview~=3.1.2
 moviepy~=1.0.3
-ipympl~=0.9.3
+ipympl~=0.9.4
 widget-bzvisualizer~=0.2.7
 widget-bandsplot~=0.7.3