get rid of some usage of "long" python2 type in pyx files

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CC:  @vbraun

Component: **refactoring**

Author: **Frédéric Chapoton**

Branch/Commit: **[`91704bb`](https://github.com/sagemath/sagetrac-mirror/commit/91704bbd12bdb01c4137d1d0dd51b68499258d26)**

Reviewer: **Kwankyu Lee**

_Issue created by migration from https://trac.sagemath.org/ticket/34739_

