MFH: r505544 print/dot2tex: Fix incorrect Python version spec

dot2tex supports Python 2 and 3. This change removes the unecessary version
spec restriction to 2.7.

While I'm here:

• Add PKGNAMEPREFIX (unique package names per flavor)
• Enable USE_PYTHON=concurrent, each package flavor installs scripts in common locations
• Add TEST_DEPENDS/test target
• Add python to CATEGORIES
• Pet portlint (Make file sort/group order)

PR: 237984
Approved by: portmgr (maintainer timeout: 1 month)

Approved by: ports-secteam (blanket(s): ports (& python) compliance)