Bump s3fs from 2021.6.0 to 2021.8.0

Bumps [s3fs](https://github.com/dask/s3fs) from 2021.6.0 to 2021.8.0.
- [Release notes](https://github.com/dask/s3fs/releases)
- [Changelog](https://github.com/dask/s3fs/blob/main/release-procedure.md)
- [Commits](fsspec/s3fs@2021.06.0...2021.08.0)

---
updated-dependencies:
- dependency-name: s3fs
  dependency-type: direct:development
  update-type: version-update:semver-minor
...

Signed-off-by: dependabot[bot] <support@github.com>

Author: dependabot[bot]

requirements_dev_optional.txt

@@ -17,6 +17,6 @@ flake8==3.9.2
 pytest-cov==2.12.1
 pytest-doctestplus==0.10.1
 h5py==3.3.0
-s3fs==2021.6.0
+s3fs==2021.8.0
 fsspec==2021.6.0
 moto[server]>=1.3.14