[requires.io] dependency update

requires · requires · commit 1281450c00fc · 2017-12-26T08:18:05.000-05:00
diff --git a/test_requirements.txt b/test_requirements.txt
@@ -1,4 +1,4 @@
 pytest==3.3.1
 pytest-cov==2.5.1
-pytest-xdist==1.20.1
+pytest-xdist==1.21.0
 hypothesis
