⬆️ Update mkdocstrings requirement from ^0.14.0 to ^0.15.0

Updates the requirements on [mkdocstrings](https://github.com/mkdocstrings/mkdocstrings) to permit the latest version.
- [Release notes](https://github.com/mkdocstrings/mkdocstrings/releases)
- [Changelog](https://github.com/mkdocstrings/mkdocstrings/blob/master/CHANGELOG.md)
- [Commits](mkdocstrings/mkdocstrings@0.14.0...0.15.0)

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

pyproject.toml

@@ -57,7 +57,7 @@ coverage = { extras = ["toml"], version = "^5.3" }
 pytest-xdist = "^2.1.0"
 pytz = "^2021.1"
 asyncmock = "^0.4.2"
-mkdocstrings = "^0.14.0"
+mkdocstrings = "^0.15.0"
 pydocstyle = "^5.1.1"
 darglint = "^1.5.4"
 ipython = "^7.16.1"