Revision 505544

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)
MFH:		2019Q2 (blanket(s): ports (& Python) compliance)