Compatibility issues with older Python versions

[ItachiSan]

Following my pull request ( #12 ), I noticed that older Python versions fail to work.

I see two options going forward:

• Adding back-compatible code for Python 2.7 and drop up to Python 3.5
• Dropping all the old versions all together

I am creating this issue so that we can have a discussion about it 😄

[dahlia]

AFAIK @spoqa, the team who maintains this project, still uses few in-house Python 2.7 projects, so the project should be compatible with Python 2.7 yet.

[ItachiSan]

The Travis CI tests however fail in all the situations for Python 2.7 and Pypy...

[ItachiSan]

@dahlia I found the issue: the package itself works, but the dependencies failed.
I've been adjusted the dependencies in order to get all to work. 😄
What about the old Python / SQLAlchemy versions?

[dahlia]

• It's okay to drop older Python 3 versions.
• Python 2.7 should be supported yet.
• SQLAlchemy 1.3+ should be supported.

[ItachiSan]

I've decided for not removing stuff from the Travis CI file but to document which versions are supported and not. 😄
Does that work? (see #12)