change python-igraph to igraph

[szhorvat]

Please see igraph/python-igraph#699 for details

Submission type

• This submission adds a new dataset
• This submission adds a new method
• This submission adds a new metric
• This submission adds a new task
• This submission adds a new Docker image
• This submission fixes a bug (link to related issue: )
• This submission adds a new feature not listed above

Testing

• This submission was written in a forked copy of openproblems
• Nextflow test pipeline is passing on this base branch of this pull
  request (include link to passed test on NF Tower found in GitHub Actions summary: )

Submission guidelines

• This submission follows the guidelines in our
  Contributing document
• I have checked to ensure there aren't other open Pull Requests for the
  same update/change

PR review checklist

This PR will be evaluated on the basis of the following checks:

• The task addresses a valid open problem in single-cell analysis
• The README provides a satisfactory task explanation (for new tasks)
• The sample test data is appropriate to test implementation of all methods and
  metrics (for new tasks)

[github-actions]

[github-actions]

This pull request has been automatically closed because it has not had recent activity.

[szhorvat]

Be aware that the python-igraph name is going away with the next version.