Remove deprecated imp

[hugovk]

Fixes #158.

imp is imported in docs/conf.py but not used, so we can delete it.

imp in setup.py is used to read the version from sox/version.py, so we can instead replace with the suggestion from the Python Packaging User Guide on "Single-sourcing the package version":

As of the release of setuptools 46.4.0, one can accomplish the same thing by instead placing the following in the project’s setup.cfg file (replacing “package” with the import name of the package):

[metadata]
version = attr: package.__version__

[lostanlen]

i have resolved the conflicts with recently merged #161 in setup.cfg. @hugovk is this good?

[coveralls]

coverage: 98.823%. remained the same
when pulling c0052cd on hugovk:replace-imp
into 5e93526 on rabitt:master.

[hugovk]

All set!