Update fasta_generate_regions.py

[jdeligt]

Minimal changes to make it python3 and 2 compatible, fixes some python pains for people installing through conda

[tseemann]

If you are using Conda, it should be using the one that comes with vcflib and is about to get a new release. But I will update the bundled one too.

[tseemann]

@jdeligt

1. #!/usr/bin/python is still python 2. needs to be #!/usr/bin/env python3