Count(xxx, distinct=True) generates incorrect sql

Basically, doing this (warning, pseudo code):

store.execute(
    Select(columns=[Count((a,b,c), distinct=True)],
    tables=[A],
    where=xxxx)
)

results in sql like:

select count(distinct a,b,c) from A where xxx

From my understanding, the above sql is not valid for Postgres, although
it works with other databases like mysql etc. What Postgres wants is this:

select count(distinct (a,b,c)) from A where xxx

Note the extra () around the column list. I think also the above may be
more correct from an sql standpoint?

At the moment, using storm's current implementation of Count() you get
the following error:

ProgrammingError: function count(integer, integer, integer, integer)
does not exist LINE 1: SELECT COUNT(DISTINCT a,b,c) ... HINT: No
function matches the given name and argument types. You might need to
add explicit type casts.

The fix is to change expr.py:

Current:
        if count.distinct:
            return "COUNT(DISTINCT %s)" % column
New:
        if count.distinct:
            return "COUNT(DISTINCT (%s))" % column