README: fix required pytest version

Ref: #603 (comment).

Author: blueyed

README.rst

@@ -30,7 +30,7 @@ pytest-django allows you to test your Django project/applications with the
 
   * Django: 1.8-1.11, 2.0 and latest master branch (compatible at the time of each release)
   * Python: CPython 2.7, 3.4-3.6 or PyPy 2, 3
-  * pytest: >2.9.x
+  * pytest: 3.6 or later
 
 * Licence: BSD
 * Project maintainers: Andreas Pelme, Floris Bruynooghe and Daniel Hahler