Pin python-igraph to latest version 0.9.11

[pyup-bot]

This PR pins python-igraph to the latest release 0.9.11.

Changelog

0.9.10

Changed

- Vendored igraph was updated to version 0.9.8.

Fixed

- Fixed plotting of curved edges in the Cairo plotting backend.

- `setup.py` now looks for `igraph.pc` recursively in `vendor/install`; this
fixes building igraph from source in certain Linux distributions

- `Graph.shortest_paths()` does not crash with zero-length weight vectors any
more

- Fix a memory leak in `Graph.delete_vertices()` and other functions that
convert a list of vertex IDs internally to an `igraph_vs_t` object, see
[503](https://github.com/igraph/python-igraph/issues/503) for details.

- Fixed potential memory leaks in `Graph.maximum_cardinality_search()`,
`Graph.get_all_simple_paths()`, `Graph.get_subisomorphisms_lad()`, 
`Graph.community_edge_betweenness()`, as well as the `union` and `intersection`
operators.

- Fix a crash that happened when subclassing `Graph` and overriding `__new__()`
in the subclass; see [496](https://github.com/igraph/python-igraph/issues/496)
for more details.

- Documentation now mentions that we now support graphs of size 5 or 6 for
isomorphism / motif calculations if the graph is undirected

Links

• PyPI: https://pypi.org/project/python-igraph
• Changelog: https://pyup.io/changelogs/python-igraph/
• Homepage: https://igraph.org/python