Pin pytest-xdist to latest version 1.23.0 (#805)

Author: pyup-bot

requirements/requirements_test.txt

@@ -4,6 +4,6 @@ pydocstyle>=2.1,<2.2
 pytest>=3.6,<3.8
 pytest-cache>=1.0,<1.1
 pytest-cov>=2.5,<2.6
-pytest-xdist>=1.22,<1.23
+pytest-xdist==1.23.0
 sphinx>=1.7,<1.8
 yapf>=0.21,<0.23