Python 3.x support for schema_salad

[manu-chroma]

Get passing test locally (tox) and on travis CI in python 3.x environment

Checklist:

• python-avro support
• use fixers like modernize and futurize to fix basic issues
• integrate six library (majority done)
• handle string literals properly: u" " or from __future__ import unicode_literals (optional for the time being)
• mypy goals
  • update to latest version of mypy for --py2 mode
  • run in python3 mode
  • now using Text inplace of unicode in existing codebase
  • Add stub files for 3rd party libraries and stdlib to typeshed/3 and typeshed/2.7: Status
  • update Makefile
• edit .travis.yml, simplify tox.ini

[manu-chroma]

@tetron @mr-c im confused between using u" " or from __future__ import unicode_literal for handling strings. currently reading up on it.
Would like your thoughts on what might be a better way to go.

[kapilkd13]

Hi, I am adding few steps from my proposal. You have implemented almost all of them, still, I hope you find this useful.

• Text versus binary data
• We would also include a test in our test suite that checks if all dependencies are compatible with Python 3 or not [dropping this since we've already used caniusepython3 to verify dependency compatibility with python3. will be doing (obvious) manual checking when adding a new dependency in the future, thus making this redundant]
• Changing source code of cwltool and schema-salad as per Python 3 standard ex. print to print(), using a script like Futurize.
• Issues like “Division operator”
• Checking for dependencies which prevent our transition to Python 3 using tools like caniusepython3.

[manu-chroma]

Hey @kapilkd13, I've updated your comment to be in checklist form so I can tick mark whatever is done. Thanks btw.

[mr-c]

Done, thanks!