Fix ignored test cases

[SuibianP]

Tests under this class seem not to be run due to pytest discovery rules, which requires classes to be prefixed with Test.

This also seems to be hiding potential problems with non-PostgreSQL database systems.

[kurtmckee]

TestDateRangeOnPostgres inherits from DateRangeTestCase.

https://github.com/SuibianP/sqlalchemy-utils/blob/patch-1/tests/types/test_date_range.py#L87

The class name needs to change in both places.

[kurtmckee]

@SuibianP, thanks for your work on this. You were right -- making the tests discoverable has indeed revealed some issues. For posterity, here's the output from one of the failing tests on Python 3.7:

AttributeError("'DateInterval' object has no attribute 'lower_inc'")

Is this something you're comfortable debugging and resolving as a part of this PR?

(By the way, the Python 3.6 deprecation failures are resolved in the most recent commits. If you rebase those should disappear and let you run the tests in Python 3.6.)

[SuibianP]

@kurtmckee This issue roots from kvesteri/intervals#49, and will be automatically solved by bumping the dependency version once that PR is merged.

On a separate note, the practice of de/serialising with __str__ and the constructor is presumably fragile by nature IMHO…