Upgrade testing tools

Author: nedbat

requirements/pytest.pip

@@ -3,9 +3,9 @@
 
 # The pytest specifics used by coverage.py
 
-pytest==3.7.1
+pytest==3.8.0
 pluggy>=0.7         # pytest needs this, but pip doesn't understand
-pytest-xdist==1.22.5
+pytest-xdist==1.23.0
 flaky==3.4.0
 mock==2.0.0
 PyContracts==1.8.3

requirements/tox.pip

@@ -1,4 +1,4 @@
 # The version of tox used by coverage.py
-tox==3.1.3
+tox==3.2.1
 # Adds env recreation on requirements file changes.
 tox-battery==0.5.1