Bump ipytree from 0.2.1 to 0.2.2

Bumps [ipytree](https://github.com/martinRenou/ipytree) from 0.2.1 to 0.2.2.
- [Release notes](https://github.com/martinRenou/ipytree/releases)
- [Changelog](https://github.com/martinRenou/ipytree/blob/master/RELEASE.md)
- [Commits](https://github.com/martinRenou/ipytree/commits)

---
updated-dependencies:
- dependency-name: ipytree
  dependency-type: direct:development
  update-type: version-update:semver-patch
...

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

requirements_dev_optional.txt

@@ -2,7 +2,7 @@
 # bsddb3==6.2.6; sys_platform != 'win32'
 lmdb==1.3.0; sys_platform != 'win32'
 # optional library requirements for Jupyter
-ipytree==0.2.1
+ipytree==0.2.2
 # optional library requirements for services
 # don't let pyup change pinning for azure-storage-blob, need to pin to older
 # version to get compatibility with azure storage emulator on appveyor (FIXME)