Bump joblib from 0.13.2 to 1.2.0 in /docs

Bumps [joblib](https://github.com/joblib/joblib) from 0.13.2 to 1.2.0.
- [Release notes](https://github.com/joblib/joblib/releases)
- [Changelog](https://github.com/joblib/joblib/blob/master/CHANGES.rst)
- [Commits](joblib/joblib@0.13.2...1.2.0)

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

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

Author: dependabot[bot]

docs/requirements.txt

@@ -5,7 +5,7 @@ numpy==1.16.5
 mendeleev==0.4.5
 pint==0.17
 nteract-scrapbook==0.2.1
-joblib==0.13.2
+joblib==1.2.0
 sphinx-py3doc-enhanced-theme==2.4.0
 nbsphinx==0.4.2
 nbconvert==5.6.1