DOCS: Use of python 2 print statement in documentation

[geo7]

There might be a good reason for this - just adding in case it's of use.

From #1198 and setup.py it seems python 2 isn't supported, but documentation here : https://www.psycopg.org/docs/advanced.html#asynchronous-notifications still uses print instead of print( ... )

The following remain in doc/:

src/usage.rst
198:attack to database servers. Before continuing, please print `this page`__ as a
410:    >>> print u, type(u)
421:    >>> print conn.encoding
426:    >>> print x, type(x), repr(x)
433:    >>> print type(x), repr(x)
445:    >>> print x, type(x), repr(x)

src/cursor.rst
295:            ...     print record

src/module.rst
171:        >>> print e.pgerror

src/advanced.rst
229:    >>> print type(point), point.x, point.y
287:    print "Waiting for notifications on channel 'test'"
290:            print "Timeout"
295:                print "Got NOTIFY:", notify.pid, notify.channel, notify.payload

[dvarrazzo]

Fixed, thank you