[snafu] isolation-level change in 2.4.2

In 2.4.2 you shuffled numeric codes for isolation levels freely,
because "everybody should be using symbolic codes".

Generally that would be true, except this case: psycopg 1.x
did not have symbolic codes, so numeric codes were
part of public API. So old code that were written
for psycopg1 will not work anymore with psycopg2.

What makes is especially bad is that such code will not fail
clearly, but instead will result in silent data corruption.
We experienced that with Skytools.

Also note that even in psycopg2, the .set_isolation_level()
is part of core API, but the constants are under psycopg2.extensions.
So even pure-2.x code may be using numerical constants.

I suggest 2 ways to fix it properly:

1) Use numeric codes out of range compared to 1.x:

ISOLATION_LEVEL_AUTOCOMMIT = 10
ISOLATION_LEVEL_READ_UNCOMMITTED = 11
ISOLATION_LEVEL_READ_COMMITTED = 12
ISOLATION_LEVEL_REPEATABLE_READ = 13
ISOLATION_LEVEL_SERIALIZABLE = 14

and give errors to old codes - that would give clear detection
that somebody is using old codes.

2) Use codes that are compatible with 1.x:

ISOLATION_LEVEL_AUTOCOMMIT = 0
ISOLATION_LEVEL_READ_UNCOMMITTED = 1
ISOLATION_LEVEL_READ_COMMITTED = 1
ISOLATION_LEVEL_REPEATABLE_READ = 2
ISOLATION_LEVEL_SERIALIZABLE = 3

the REP_READ=2, SER=3 change is deliberate, because before 9.1
they were equal, and in 9.1+ the REP_READ corresponds
to old SER. The SER level in 9.1 is new, and unlikely
to be expected by old code.

I would suggest even releasing 2.4.4 quite soon with
this fixed, to avoid anybody else tripping on this bug.

--
marko