Bump future from 0.18.2 to 0.18.3 in /docs (#454)

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
...

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docs/requirements.txt

@@ -24,7 +24,7 @@ ipython==7.16.3
 ipython-genutils==0.2.0
 jedi==0.17.2
 MarkupSafe==2.0.1
-future==0.18.2
+future==0.18.3
 pillow==5.4.1
 alabaster==0.7.12
 commonmark==0.8.0