Update documented pytest requirements changed in d9a9e9a

Closes #615.

Author: Code0x58

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: 3.6 or later
+  * pytest: >=3.6
 
 * Licence: BSD
 * Project maintainers: Andreas Pelme, Floris Bruynooghe and Daniel Hahler

docs/changelog.rst

@@ -20,6 +20,11 @@ Bug fixes
 * Fixed scope of overridden settings with live_server fixture: previously they
   were visible to following tests (#612).
 
+Compatibility
+^^^^^^^^^^^^^
+
+* The required `pytest` version changed from >=2.9 to >=3.6.
+
 3.2.1
 -----