Clean up JSON workarounds for unsupported Python versions

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.

lib/_json.py

@@ -27,22 +27,13 @@ extensions importing register_json from extras.
 # FITNESS FOR A PARTICULAR PURPOSE.  See the GNU Lesser General Public
 # License for more details.
 
+import json
 import sys
 
 from psycopg2._psycopg import ISQLQuote, QuotedString
 from psycopg2._psycopg import new_type, new_array_type, register_type
 
 
-# import the best json implementation available
-if sys.version_info[:2] >= (2, 6):
-    import json
-else:
-    try:
-        import simplejson as json
-    except ImportError:
-        json = None
-
-
 # oids from PostgreSQL 9.2
 JSON_OID = 114
 JSONARRAY_OID = 199
@@ -67,13 +58,7 @@ class Json(object):
     def __init__(self, adapted, dumps=None):
         self.adapted = adapted
         self._conn = None
-
-        if dumps is not None:
-            self._dumps = dumps
-        elif json is not None:
-            self._dumps = json.dumps
-        else:
-            self._dumps = None
+        self._dumps = dumps or json.dumps
 
     def __conform__(self, proto):
         if proto is ISQLQuote:
@@ -86,13 +71,7 @@ class Json(object):
         provided in the constructor. You can override this method to create a
         customized JSON wrapper.
         """
-        dumps = self._dumps
-        if dumps is not None:
-            return dumps(obj)
-        else:
-            raise ImportError(
-                "json module not available: "
-                "you should provide a dumps function")
+        return self._dumps(obj)
 
     def prepare(self, conn):
         self._conn = conn
@@ -181,9 +160,6 @@ def register_default_jsonb(conn_or_curs=None, globally=False, loads=None):
 def _create_json_typecasters(oid, array_oid, loads=None, name='JSON'):
     """Create typecasters for json data type."""
     if loads is None:
-        if json is None:
-            raise ImportError("no json module available")
-        else:
         loads = json.loads
 
     def typecast_json(s, cur):