lift restrictions of dendropy, which is a dependency of SEPP

[sjanssen2]

As suggested by @aliciaaevans in #47895, I here lift the pinning of dendropy for PASTA and SEPP.

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[bgruening]

and this problem is now gone? The missing _convert_node_to_root_polytomy?

[sjanssen2]

According to smirarab/sepp#126, which has been merged in the current release, the issue should have been solved.