Removal of implicit dependency on py

`pytest-xdist` depends on `pytest>=3`.
`pytest 3.0.0` requires `py>=1.4.29`
and latest pytest requires `py>=1.4.33`.
Thus it is unnecessary for pytest-xdist to include
a requirement on `py>=1.4.22`

Fixes pytest-dev#105

changelog/105.bugfix

@@ -0,0 +1 @@
+Removal of implicit dependency on incorrect version of py.

setup.py

@@ -20,7 +20,7 @@
         ],
     },
     zip_safe=False,
-    install_requires=['execnet>=1.1', 'pytest>=3.0.0', 'py>=1.4.22'],
+    install_requires=['execnet>=1.1', 'pytest>=3.0.0'],
     setup_requires=['setuptools_scm'],
     classifiers=[
         'Development Status :: 5 - Production/Stable',