0.12.6beta1: Print a warning when setuptools version >= 5.4 and < 5.7 are detected. Refs #11694.

Patch by Jun Omae.


git-svn-id: http://trac.edgewall.org/intertrac/log:/branches/0.12-stable@13115 af82e41b-90c4-0310-8c96-b1721e28e2e2