Unpin requirements
Remove version import from setup.py

The `import requests` statement results in an ImportError when
installing in a fresh environment. Importing the module or anything from
it requires importing requests which isn't installed until setup has
installed requests.
Test with tox
Further simplify the package

The core module required the `__version__` attribute, which really
belongs in the `__init__.py` module file, but the latter imports from
core.py so that would introuce a circular import. The attribute could be
defined in the core.py file and imported into __init__.py, but this
reduces our ability to fetch the version attribute through means other
than Python imports.

Since this is effectively a one file module anyhow, flattening the
module is a good solution.