Bump monty from 2021.7.8 to 2021.12.1

Bumps [monty](https://github.com/materialsvirtuallab/monty) from 2021.7.8 to 2021.12.1.
- [Release notes](https://github.com/materialsvirtuallab/monty/releases)
- [Changelog](https://github.com/materialsvirtuallab/monty/blob/master/docs/changelog.html)
- [Commits](materialsvirtuallab/monty@v2021.7.8...v2021.12.1)

---
updated-dependencies:
- dependency-name: monty
  dependency-type: direct:development
  update-type: version-update:semver-minor
...

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

Author: dependabot[bot]

requirements-dev.txt

@@ -1,6 +1,6 @@
 invoke==1.4.1
 requests==2.23.0
-monty==2021.7.8
+monty==2021.12.1
 black==21.11b1
 flake8==3.8.2
 isort==4.3.21