Currently, inspect.isabstract() may return an integer like 1048576 
instead of True, or 0 instead of False.

Although technically correct, no other isXXX function behaves that way; 
also, isgeneratorfunction() uses a similar code construct but casts the 
result into a boolean.

The attached patch makes inspect.isabstract() return a boolean value 
always.

The patch works, as this is what is implicitly happening anyway if you use 
the function. There seem to be no tests for this function, so there is 
nothing to break. I guess this is the right time to get some tests. 

Gabriel, would you like to write tests for this function?

Tests added.

I changed the patch so it does not introduce new dependencies and fails 
before the patch of isabstract().

I don't think we should test the actual type returned (bool); that 
would be overspecifying. The documentation doesn't promise it IS a 
boolean, and no other test for any isXXX() function checks the type 
either.
I'd remove the last five assertions.

That's fine with me. Looks like nobody wants to check it in anyways.

Applied in r75433.