Issue 1904: Add key argument to heapq functions

Wouldn't it be useful to add a "key" argument to some heapq functions ?
So you could construct a heap from dict items list based on the second
item of each tuple, for example.

Adding a simple patch that adds the "key" argument. To test this you
need to comment lines where it tries to import the C version of heapq.

Rejecting the key-function patch in favor of the current typical 
approach of heapifying tuples that link a priority level with 
individual records.  I've been a heavy user of the module and one of 
its principal maintainers.  To date, I've seen very little need for key-
function argument.

Use cases aside, there is another design issue in that the key-function 
approach doesn't work well with the heap functions on regular lists.  
Successive calls to heap functions will of necessity call the key-
function multiple times for any given element.  This contrasts with sort
() where the whole purpose of the key function was to encapsulate the 
decorate-sort-undecorate pattern which was desirable because the key-
function called exactly once per element.