Under Python 3, `scipy.sparse.coo_matrix` fails when passed memoryviews.
We coerce the memoryviews to numpy arrays before passing them to the
`coo_matrix`. Even though this is kind of a hack it shouldn't be very
inefficient as no data is actually copied. See
https://github.com/scipy/scipy/issues/5123 for details.

Unbound methods are not longer a thing in Python 3 [1].  Instead,
functions use the descriptor protocol [2] to properly bind the `self`
argument if they are being called as a method. Cython functions do not
have a `__get__` attribute and so don't satisfy the descriptor
protocol. They therefore can't be used as methods. As a workaround we
create a wrapper class with the proper __get__ method. The advantage of
this over creating a Python wrapper is that the function signature is
preserved.

[1]: https://docs.python.org/3.0/whatsnew/3.0.html
[2]: https://docs.python.org/3/howto/descriptor.html

Also, replace redundant check against Callable abc
with builtin `callable` function.

The higher-level wrappers like `decomp_lu` take
strings, and we convert them explicitly inside.

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.

We do not use it, but this makes sure that it's included in SOURCES.txt.  This
is necessary so that source distributions can be recreated identically from
source distributions.

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.