DELF: Fix CSV read error

[khatchad]

Fix https://github.com/tensorflow/models/issues/10474. CSVs should be text files.

Type of change

• [X] Bug fix (non-breaking change which fixes an issue)

Tests

I followed the instructions at https://github.com/tensorflow/models/blob/b4c7ea90e8e9209c568d1b59f71432cfd69bb688/research/delf/delf/python/training/README.md#prepare-the-data-for-training.

Checklist

• [X] I have signed the Contributor License Agreement.
• [X] I have read guidelines for pull request.
• [X] My code follows the coding guidelines.
• [X] I have performed a self code review of my own code.
• [X] I have commented my code, particularly in hard-to-understand areas.
• [X] My changes generate no new warnings.