install_requires: do not allow pytest 4.2.0 (#697)

Ref: pytest-dev/pytest#4704

Author: blueyed

setup.py

@@ -31,7 +31,7 @@ def read(fname):
     python_requires='>=2.7, !=3.0.*, !=3.1.*, !=3.2.*, !=3.3.*',
     setup_requires=['setuptools_scm>=1.11.1'],
     install_requires=[
-        'pytest>=3.6',
+        'pytest>=3.6,!=4.2.0',
         'pathlib2;python_version<"3.4"',
     ],
     extras_require={
@@ -40,7 +40,6 @@ def read(fname):
             'sphinx_rtd_theme',
         ],
         'testing': [
-            'pytest>=3.6',
             'Django',
             'django-configurations>=2.0',
             'pytest-xdist>=1.15',