Bump imageio-ffmpeg from 0.4.7 to 0.4.9

Bumps [imageio-ffmpeg](https://github.com/imageio/imageio-ffmpeg) from 0.4.7 to 0.4.9.
- [Release notes](https://github.com/imageio/imageio-ffmpeg/releases)
- [Commits](imageio/imageio-ffmpeg@v0.4.7...v0.4.9)

---
updated-dependencies:
- dependency-name: imageio-ffmpeg
  dependency-type: direct:production
  update-type: version-update:semver-patch
...

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

requirements_docs.txt

@@ -3,7 +3,7 @@ cmocean==3.1.3
 colorcet==3.0.1
 geovista==0.3.0
 imageio>=2.5.0
-imageio-ffmpeg==0.4.7
+imageio-ffmpeg==0.4.9
 ipygany==0.5.0
 jupyter_sphinx==0.4.0
 jupyterlab==4.1.2