Bump future from 0.16.0 to 0.18.3

Bumps [future](https://github.com/PythonCharmers/python-future) from 0.16.0 to 0.18.3.
- [Release notes](https://github.com/PythonCharmers/python-future/releases)
- [Changelog](https://github.com/PythonCharmers/python-future/blob/master/docs/changelog.rst)
- [Commits](PythonCharmers/python-future@v0.16.0...v0.18.3)

---
updated-dependencies:
- dependency-name: future
  dependency-type: direct:production
...

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

requirements.txt

@@ -3,4 +3,4 @@ mock==1.3.0
 nose==1.3.7
 voluptuous==0.8.10
 Flask==0.10.1
-future==0.16.0
+future==0.18.3