Bump ipytree from 0.1.3 to 0.2.1 (#744)

Bumps [ipytree](https://github.com/martinRenou/ipytree) from 0.1.3 to 0.2.1.
- [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)

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

Co-authored-by: dependabot[bot] <49699333+dependabot[bot]@users.noreply.github.com>

Author: dependabot[bot]

requirements_dev_optional.txt

@@ -2,7 +2,7 @@
 # bsddb3==6.2.6; sys_platform != 'win32'
 lmdb==0.97; sys_platform != 'win32'
 # optional library requirements for Jupyter
-ipytree==0.1.3
+ipytree==0.2.1
 # 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)