Update PyTest to v5.x to avoid attr incompatibility. (#2428)

* recent 19.2.0 attr release is incompatible with our version of pytest

* update pytest to v5

Author: crwilcox

testing/requirements.txt

@@ -6,7 +6,7 @@ mock==2.0.0
 mysql-python==1.2.5; python_version < "3.0"
 PyCrypto==2.6.1
 pytest-cov==2.6.1
-pytest==4.2.0
+pytest==5.2.0
 pyyaml==3.13
 responses==0.10.5
 WebTest==2.0.32