When setuptools imports from Cython (Python 3.5),
setuptools.command.build_ext is defined,
but setuptools.command.build_ext has no attribute .build_ext, yet,
causing the `_build_ext.build_ext` to fail with an AttributeError.

By attempting to import the object, rather than its containing module, the check is more thorough.

This allows extending from it with a cdef class. Closes #724.

Do not warn for "cache" option

This fixes #1473.

Use create_typedef_type instead of creating directly

Fix refcounts for transposed memoryview slices.

Strangely enough, this test compiles, but I am able to reproduce this
error outside the testing environment.  Still worth having here.

Added more detailed documentation for using cygdb

Cython is still built and installed with the standard distutils
setup.py mechanisms, but this allows other bazel projects to
reference and use Cython as part of their build process.

try to work around a Py_FatalError crash in latest Py3.6 apparently due to a RuntimeWarning during garbage collection

Replace old build_ext with new cythonize(...) version.

This closes #1433.

p_f_string_expr produced warning: implicit conversion loses integer precision: 'long' to 'Py_UCS4' (aka 'unsigned int') [-Wshorten-64-to-32]

Closes #1458.

Support dependencies in conditional compilation