The Mapping_update() method iterated over a sequence of 2-tuples, but
held an extra reference to the sequence.  The sequence object was
DECREFed on an error exit but not on the normal return.

The fix reworks the control flow of the function to make the reference
counting behavior a little clearer:

   - Test for call with no arguments and return immediately if so.

   - If the argument is a sequence, don't INCREF it.

   - If the argument passed isn't a sequence, call its items() method
     and store the result in the local variables seq and items.

   - On exit, do Py_XDECREF(items).  This will do a decref only if a
     the second case above applied.

Two other minor nits:

   - Make consistent use of spaces around = in assignments (as opposed
     to a fix of both)

   - Remove assignment from conditional expression.