improve deadlock detection for various versions of psycopg2

improve deadlock detection for various versions of psycopg2

Psycopg2 has changed the kind of exception that is emitted on
deadlocks between versions 2.7 and 2.8. The code was already
trying to catch both kind of errors but because the
psycopg2.errors package is unknown in 2.7 and below, the
code would throw an exception on anything but a deadlock error.

This commit wraps the deadlock handling into a context manager
to avoid code duplication and uses module imports to detect if
the new error codes are available.

Also sets the required psycopg2 version to 2.7 or bigger as
versions below are difficult to test.