Python 3 Update

[EdwinGuzman]

Fixes SIMPLY-2215.

The goal for this is to fully convert the codebase to python 3 (3.6 which seems stable on Travis). This is used as a git submodule so Docker is not needed for this repo.

• Ran the codebase through 2to3.
• Updated all TODO python3 comments
• Removed dependency on six and futures.
• Removed native_string helper function.
• A common update was moving from urlparse to urllib.parse.
• Updated how metaclass is declared.
• Had to update some map/filter functions to list comprehensions.
• Had to fix comparisons where one variable could be None and it was expecting a number.
• Updated lxml tounicode to tostring and slightly updated the call for the updated package version.

Missing todos:

• A migration script to update the marc_record Work column to binary strings. Moving from String to Binary in the database works but as discussed with Leonard, this may be a big migration.

Although this PR is using python 3, there are still updates I discussed with @nballenger that are not part of this PR but want to include in subsequent PRs such as:

• linting
• moving from nosetests to pytest
• code coverage tool

[vbessonov]

I haven't finished walking through the whole PR yet, I wanted to add some comments but accidentally submitted an approval.

[EdwinGuzman]

This PR now includes the pytest update done by @jonathangreen . I switched from python 2.7 to python 3.6 in the tox.ini file and updated a few files with minor updates (the biggest/tedious part of this was fixing merge conflicts).