Bump dockerflow from 2020.10.0 to 2021.7.0

Bumps [dockerflow](https://github.com/mozilla-services/python-dockerflow) from 2020.10.0 to 2021.7.0.
- [Release notes](https://github.com/mozilla-services/python-dockerflow/releases)
- [Changelog](https://github.com/mozilla-services/python-dockerflow/blob/main/docs/changelog.rst)
- [Commits](mozilla-services/python-dockerflow@2020.10.0...2021.7.0)

---
updated-dependencies:
- dependency-name: dockerflow
  dependency-type: direct:production
  update-type: version-update:semver-major
...

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

requirements.txt

@@ -10,7 +10,7 @@ colander==1.8.3
 colorama==0.4.4
 cornice==5.2.0
 cornice-swagger==1.0.0
-dockerflow==2020.10.0
+dockerflow==2021.7.0
 ecdsa==0.17.0
 hupper==1.10.3
 idna==2.10