math/py-igraph: Update to 0.10.6

* Switch to DISTVERSION
* Always use Flex from ports
* Use available dependencies in tree instead of vendored
  This is also consistent with math/igraph

Changelog:
https://github.com/igraph/python-igraph/releases/tag/0.10.6

PR:		273168
Reviewed by:	lwhsu (maintainer)
Sponsored by:	Blinkinblox