v0.6.0

• New, more coherent C API. This change should be transparent to users
  of the Python module.
• Add variance uncertainty parameters errx2, erry2 and errxy to
  output of sep.extract().
• Add a minimum sigma to sep.winpos() to match Source Extractor
  behavior.
• Fix use of boolean masks in sep.kron_radius(). Formerly, using a
  boolean mask resulted in nonsense results.
• Fix segfault in Background.back() when box size is same as image size.
• Fix bug in creating long error messages on Python 3.