AVRO-2657: Fix Python Interop Data Generator

[kojiromike]

Jira

• My PR addresses AVRO-2657
• ...and references it in the PR title.
• ...adds no dependencies.

Tests

• Fixes where build.sh puts the interop test data generated by Python.

Commits

• My commits all reference Jira issues in their subject lines.
• My commits follow the guidelines from "How to write a good git commit message"

Documentation

• N/A

[kojiromike]

@sekikn Please review.

[sekikn]

Looks good for the interop-data-generate function :) In addition, interop-data-test should be also fixed so that it can refer to the files generated by other language bindings.

[kojiromike]

I've decided to roll this into #742 because I want to use tox itself to make the interop avro files available in the tox virtualenvs for each python implementation.