Repair canonical_transcripts

[EFraim]

Canonical transcripts was not functional with newer (>3.5) pythons and newer versions of the class feature (it assumes raw string representation of sequences, which is no longer the case)
I've verified the results against current Ensembl gff database.

Pull request #156
addresses the issue.
It also apparently breaks 2.x compatibility. Is this of concern to the project?

[daler]

Now addressed in v0.11.

[EFraim]

Thanks!