dataclasses is >= Python 3.7 need to update requirements.txt to support 3.6

[dlazesz]

As line 6 in embert/data_classes.py imports dataclasses which requires Python 3.7 minimum the line dataclasses needed to be added to requirements.txt for Pyhton 3.6 support.

Please update requirements.txt or correct the supported Python versions in setup.py!

Thank you!

[DavidNemeskey]

Fixed in #6.