Now we exec _kwant_version.py into a namespace and get
the version from that.

Specify only Python3 support in trove classifiers.
https://www.python.org/dev/peps/pep-0301/#distutils-trove-classification

Python 3 no longer allows implicit relative imports:
https://docs.python.org/release/3.0.1/whatsnew/3.0.html#removed-syntax
Instead of importing the version file we just parse it. By keeping
the extension ".py" we make it so that it will be distributed along
with the rest of the package.

The pip tool messes up the timestamps of files, so with our previous strategy
of enabling cython by default the build fails if Cython is not available.

Also, turn the "warning" into a "caution" message that is less frightening and
send it to stdout only, so that it does not appear with pip.

This offers better dependency handling (the compilation of some .c files
depends on build.conf, but their cythonization doesn't), and other advantages
like parallel builds.

When the --no-cython option is given, it is no longer an error when
derived files are older than source files.

Error messages have been improved.

The file kwant/_static_version.py is no longer written upon every
invocation of setup.py.  Instead, a file kwant/_kwant_version.py is
injected into source and binary distributions.

The static version in kwant/_kwant_version.py now takes precedence
before any version from git.  This makes the --no-git option to setup.py
no longer necessary, since the existence of kwant/_kwant_version.py has
the same role.  The motivation behind this change is to allow keeping
Kwant distributions in git repositories that have a different layout
from the canonical Kwant git repository.  This is useful for example for
packaging.

The name of the static version file was changed to avoid problems with
stale files.

This is useful when the source is held in a git repository but this
repository is not an usual kwant repository.