replace py.test module references with pytest

The only remaining 'py.test' references are:
 * those referring to the 'py.test' executable
 * those in code explicitly testing py.test/pytest module compatibility
 * those in old CHANGES documentation
 * those in documentation generated based on external data
 * those in seemingly unfinished & unmaintained Japanese documentation

Minor stylistic changes and typo corrections made to documentation next to
several applied py.test --> pytest content changes.

doc/en/example/assertion/failure_demo.py

@@ -1,4 +1,4 @@
-from py.test import raises
+from pytest import raises
 import py
 
 def otherfunc(a,b):