Migrate "organisms" and "genes" apps to Python 3

[dongbohu]

genes/models.py is copied from django-genes and converted to Python 3 by 2to3 command. @mhuyck raised some good questions when reviewing the PR #1:
https://github.com/greenelab/py3-adage-backend/pull/1/files#diff-c72e57d6973b30333ee5ea17efb052ac

The other issues for the migration includes:

• Migrate genes/tests.py to Python 3
• Migrate management commands in django-genes to Python 3
• genes/api.py

[dongbohu]

APIs that are supported in original django-genes:

• genes: https://github.com/greenelab/tribe/blob/master/docs/source/api.rst#genes-endpoint
  • xrid_translate: https://github.com/greenelab/tribe/blob/master/docs/source/api.rst#tribe-translate
• crossrefdb: https://github.com/greenelab/tribe/blob/master/docs/source/api.rst#cross-reference-databases-endpoint

[dongbohu]

All modules in organisms have been ported to Python 3.

Most modules in genes have been ported to Python 3, except the following two management commands that were written for Tribe:

• genes_load_uniprot.py
• genes_load_wb.py

To-do list when porting Tribe to Python 3:

• Porting the two management commands above
• Test other modules of organisms and genes in Tribe
• Package organisms into django-organisms, genes into django-genes, and publish them in PyPI. See the latest instructions at:
  • Django doc (How to write reusable apps): https://docs.djangoproject.com/en/3.0/intro/reusable-apps/
  • Packaging Python Projects: https://packaging.python.org/tutorials/packaging-projects/