Upgrade pyelliptic to version 1.5.5.

### Motivation
The [new](https://github.com/yann2192/pyelliptic/compare/1.5.4...1.5.5) release of `pyelliptic` ships under BSD license and fixes a possible security issue. Both are appreciated.

### Changelog
* Upgrade `pyelliptic` to version `1.5.5`