setup: remove unneeded import

We don't build a C extension anymore.
README: add some of those nifty useless badges
README: update the license to say that this is ISC license.

And update the text to use and/or wording which seems to be less
ambiguous.
copyright: don't use a box of comments
setup: move some metadata inside snimpy module
pep8: be PEP8 compliant
tests: unshadow two tests
tests: don't display banner during tests

We intercept `InteractiveConsole.write()` to do nothing.
dev: provide a requirements.txt file
setup: remove unneeded `check_output()` compat function
doc: use Sphinx to generate documentation

We just convert `README.md` to a proper documentation. We keep a
restructuredText `README.rst` file and use it with the history for the
long description on Pypi.
docs: add a section about licensing

Morph the author section into a licensing section.
docs: provide an API documentation

The documentation is incomplete but sufficient for most needs. Most of
the API is not stable anyway.