Bump future from 0.17.1 to 0.18.3 in /ci

Bumps [future](https://github.com/PythonCharmers/python-future) from 0.17.1 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@v0.17.1...v0.18.3)

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

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

Author: dependabot[bot]

ci/requirements-tests.txt

@@ -10,7 +10,7 @@ attrs==19.1.0
 cffi==1.12.2
 coverage==4.5.3           # via pytest-cov
 execnet==1.5.0            # via pytest-cache
-future==0.17.1
+future==0.18.3
 mccabe==0.6.1             # via pytest-mccabe
 more-itertools==5.0.0     # via pytest
 numpy==1.16.2