Bump aniso8601 from 7.0.0 to 8.1.0

Bumps [aniso8601](https://bitbucket.org/nielsenb/aniso8601) from 7.0.0 to 8.1.0.
- [Changelog](https://bitbucket.org/nielsenb/aniso8601/src/master/CHANGELOG.rst)
- [Commits](https://bitbucket.org/nielsenb/aniso8601/branches/compare/v8.1.0..v7.0.0)

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

Author: dependabot[bot]

requirements_all.txt

@@ -1,5 +1,5 @@
 alembic==1.4.3
-aniso8601==7.0.0
+aniso8601==8.1.0
 apache-airflow==1.10.12
 apispec==1.3.3
 appdirs==1.4.4