MNT: (deps): Bump dask from 2021.2.0 to 2021.8.1 in /ci
MNT: (deps): Bump dask from 2021.2.0 to 2021.8.1 in /ci

Bumps [dask](https://github.com/dask/dask) from 2021.2.0 to 2021.8.1.
- [Release notes](https://github.com/dask/dask/releases)
- [Changelog](https://github.com/dask/dask/blob/main/docs/release-procedure.md)
- [Commits](https://github.com/dask/dask/compare/2021.02.0...2021.08.1)

---
updated-dependencies:
- dependency-name: dask
  dependency-type: direct:production
  update-type: version-update:semver-minor
...

Signed-off-by: dependabot[bot] <5bdcd3c0d4d24ae3e71b3b452a024c6324c7e4bb@github.com>
Unidata/MetPy,Unidata/MetPy