py-graph-tool: Update to version 2.91

[essandess]

Description

Type(s)

• bugfix
• enhancement
• security fix

Tested on

macOS x.y
Xcode x.y / Command Line Tools x.y.z

Verification

Have you

• followed our Commit Message Guidelines?
• squashed and minimized your commits?
• checked that there aren't other open pull requests for the same change?
• referenced existing tickets on Trac with full URL in commit message?
• checked your Portfile with port lint?
• tried existing tests with sudo port test?
• tried a full install with sudo port -vst install?
• tested basic functionality of all binary files?
• checked that the Portfile's most important variants haven't been broken?

[essandess]

This PR is ready to merge.

The CI errors here arise because there is no way to install boost dependencies with specific Python variants. This is unrelated to the port py-graph-tool.

Error: Failed to configure py311-graph-tool: boost181 must be installed with +python311