removed dataclasses dependency

[micah-williamson]

Dacite is requiring dataclasses a dependency. Dataclasses is a root python module and is included with python. Using version 0.0.25 may result in including a version of dataclasses that is incompatible with the most recent version of the typings module.

if sys.version_info < (3, 7):
    requirements = ["dataclasses"]
else:
    requirements = []

The python version check here was meant to only include dataclasses if the python version was <3.7.0. But doing this version check in pip is unreliable, and results in including the dataclasses package even when using python >=3.7.0.

Attempts to use python3.7 -m pip does not work.

These lines should be removed. Anyone using python <3.7.0 must pin to an older version of dacite or upgrade.

[konradhalas]

Fixed in a different way (please check current .travis.yml config), but probably still I need add info that if you want to install it from PyPi you have to install dataclasses manually.