Bump mkdocstrings from 0.13.6 to 0.15.2

Bumps [mkdocstrings](https://github.com/mkdocstrings/mkdocstrings) from 0.13.6 to 0.15.2.
- [Release notes](https://github.com/mkdocstrings/mkdocstrings/releases)
- [Changelog](https://github.com/mkdocstrings/mkdocstrings/blob/master/CHANGELOG.md)
- [Commits](mkdocstrings/mkdocstrings@0.13.6...0.15.2)

---
updated-dependencies:
- dependency-name: mkdocstrings
  dependency-type: direct:production
  update-type: version-update:semver-minor
...

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

requirements.txt

@@ -4,8 +4,6 @@
 #
 #    pip-compile
 #
-beautifulsoup4==4.9.3
-    # via mkdocstrings
 certifi==2020.6.20
     # via
     #   matplotlib
@@ -34,7 +32,9 @@ importlib-metadata==4.6.1
     #   markdown
     #   pep517
 jinja2==2.11.3
-    # via mkdocs
+    # via
+    #   mkdocs
+    #   mkdocstrings
 joblib==0.17.0
     # via nltk
 kiwisolver==1.2.0
@@ -48,24 +48,31 @@ lz4==3.1.3
 markdown==3.3.2
     # via
     #   mkdocs
+    #   mkdocs-autorefs
     #   mkdocs-material
+    #   mkdocstrings
     #   pymdown-extensions
 markupsafe==1.1.1
-    # via jinja2
+    # via
+    #   jinja2
+    #   mkdocstrings
 matplotlib==3.3.2
     # via seaborn
 mkdocs==1.1.2
     # via
     #   -r requirements.in
+    #   mkdocs-autorefs
     #   mkdocs-material
     #   mkdocstrings
+mkdocs-autorefs==0.2.1
+    # via mkdocstrings
 mkdocs-material==6.1.0
     # via
     #   -r requirements.in
     #   mkdocs-material-extensions
 mkdocs-material-extensions==1.0.1
     # via mkdocs-material
-mkdocstrings==0.13.6
+mkdocstrings==0.15.2
     # via -r requirements.in
 nltk==3.5
     # via lunr
@@ -137,8 +144,6 @@ six==1.15.0
     #   lunr
     #   pyopenssl
     #   python-dateutil
-soupsieve==2.0.1
-    # via beautifulsoup4
 sqlalchemy==1.3.20
     # via opensafely-cohort-extractor
 sqlparse==0.4.1