Support Biopython 1.68 with sub_features workaround

Biopython deprecated and removed sub_features but the GFF code has not
yet put in proper fixes. This works around the issue by adding
sub_features when missing so GFF output still works correctly
with Biopython >= 1.68.

Fixes chapmanb#110. Fixes biopython/biopython#928

v0.6.2: update version for Python 3 compatibility fixes chapmanb#96

v0.6.1: Provide Python3 compatibility fixes from @TheOneHyer. Also in…

…clude in GFFParser and use six compatibility library for urllib issues. Fixes chapmanb#90

v0.6: Handle trans-splicing GFF cases where child locations may not m…

…atch back to parents. Fixes chapmanb#91

bcbio-gff v0.4

Version 0.6.0 release plus move into own bcbio-nextgen repository