[IMP] flanker: use newer syntax compatible with pip >= 25.1

[ap-wtioit]

In our images we run the newest pip, and the flanker install failed with the old url syntax for pip 26.0.

This is not strictly necessary for current doodba but i think it would be fine to have the new syntax for flanker install and be ready for the future. (15.0 still uses the old flanker (0.9.11) from pypi and does not have the url issue)

Output from local venv when installing flanker with pip install "git+https://github.com/mailgun/flanker.git@v0.9.15#egg=flanker[validator]"

error: invalid-egg-fragment

× The 'flanker[validator]' egg fragment is invalid
╰─> from 'git+https://github.com/mailgun/flanker.git@v0.9.15#egg=flanker[validator]'

hint: Try using the Direct URL requirement syntax: 'name[extra] @ URL'

pip install "flanker[validator] @ git+https://github.com/mailgun/flanker.git@v0.9.15" works at least with pip >= 23.0.1 (16.0.Dockerfile)

See also pypa/pip#13157

Info @wt-io-it