BUG: Homogenezie nnz type to be int for all sparse matrix types.

Now all nnz types are `int`, this fixes [issue 2574](https://github.com/scipy/scipy/issues/2574)
```
format = coo
getnnz type = <class 'int'>

format = bsr
getnnz type = <class 'int'>

format = csc
getnnz type = <class 'int'>

format = csr
getnnz type = <class 'int'>

format = dia
getnnz type = <class 'int'>

format = dok
getnnz type = <class 'int'>

format = lil
getnnz type = <class 'int'>
```