[requires.io] dependency update

requires · requires · commit 2a97c0a53afa · 2017-12-26T08:17:36.000-05:00
diff --git a/test_requirements.txt b/test_requirements.txt
@@ -1,5 +1,5 @@
 pytest==3.3.1
 pytest-cov==2.5.1
 coverage==4.4.2
-pytest-xdist==1.20.1
+pytest-xdist==1.21.0
 hypothesis
