Bump future from 0.18.2 to 0.18.3 in /doc/.sphinx

Bumps [future](https://github.com/PythonCharmers/python-future) from 0.18.2 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@v0.18.2...v0.18.3)

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

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

doc/.sphinx/requirements.txt

@@ -15,7 +15,7 @@ sphinxemoji==0.1.6
 sphinx_markdown_tables==0.0.15
 CommonMark==0.9.1
 docutils==0.16
-future==0.18.2
+future==0.18.3
 idna==2.9
 imagesize==1.2.0
 Jinja2==2.11.3