The docs don't need to describe what will happen on Python versions
before 2.6 as they are unsupported by psycopg2.
Should have been included in commit
d58844e548, but was missed.
All Python versions supported by psycopg2 have the json module. It was
added in Python 2.6. Can remove checks for availability, slightly
simplifying the code.
Pass a dumps function instead. Allow customizing by either arg passing or
subclassing.
The basic Json class now raises ImportError on getquoted() if json is not
available, thus allowing using a customized Json subclass even when the json
module is not available.