Moving forward with SecretStorage and D-Bus bindings

Hi @jaraco,



One of the major issues with Keyring on Linux is the [dbus-python](https://pypi.python.org/pypi/dbus-python) dependency, which has the following problems:



- It is difficult to install from PyPI: there are no wheels, it needs the D-Bus headers installed locally and compiles itself during install.

- It is not actively developed, and the author [recommends](https://pypi.python.org/pypi/dbus-python#problems-and-alternatives) switching to another library.



Some time ago I created a port of SecretStorage to [Jeepney](https://gitlab.com/takluyver/jeepney), a modern pure-Python library by @takluyver. Please see mitya57/secretstorage#10 and the [wip/jeepney](https://github.com/mitya57/secretstorage/tree/wip/jeepney) branch. I want it to eventually become the new release of SecretStorage.



But Jeepney only supports Python 3.5+, while Keyring aims to support Python 2. So I wonder how we should resolve this issue. I see the following possibilities:



1. Release the branch as a separate project, like `secretstorage-ng`. Tweak `setup.py` to select `secretstorage-ng` for newer Pythons and `secretstorage` for older Pythons. The module name will not change, so `secretstorage` and `secretstorage-ng` will conflict with each other.

2. Release the branch as `secretstorage 3.0a1`. PyPI will not pull it automatically, but `setup.py` can select `secretstorage>=3.0a1` for newer Pythons.

3. Release the branch as `secretstorage 3.0`. Make `setup.py` select `secretstorage<3` for older Pythons. I think I prefer this option, but I may consider others too.

4. Try to fork Jeepney and add Python 2 / 3.4 support?



What do you think?

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