Bump pytest-xdist from 1.29.0 to 1.31.0

Bumps [pytest-xdist](https://github.com/pytest-dev/pytest-xdist) from 1.29.0 to 1.31.0.
- [Release notes](https://github.com/pytest-dev/pytest-xdist/releases)
- [Changelog](https://github.com/pytest-dev/pytest-xdist/blob/master/CHANGELOG.rst)
- [Commits](pytest-dev/pytest-xdist@v1.29.0...v1.31.0)

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

requirements-test.txt

@@ -6,5 +6,5 @@ mirakuru<2.1.0; python_version<='3.5'
 mysqlclient==1.4.6 # rq.filter: !=1.4.2.post1
 port-for==0.4
 pytest-cov==2.8.1
-pytest-xdist==1.29.0
+pytest-xdist==1.31.0
 mock==3.0.5