Pp stash constrain

Bugfix for problem introduced by pp constraint 'conversion' code in v1.7.
Various users had code with the idiom ```constraint = AttributeConstraint(STASH=STASH(x,y,z))```.
When used to load, 'pp._convert_constraints' refused to handle these, as it expected the test value to be a string (only).  Fixed that by supporting a STASH object in the convert code.
Fixed as for the previous case (https://github.com/SciTools/iris/pull/1306/files#r17301404)