allow configuring the source language level in pyximport use default to the platform version for .py files

fix crashes when deallocating instances of cimported types after module cleanup, patch by Lisandro Dalcin

cython_inline: Stop passing unused `ctx` parameter to `cythonize`.

Update docs/src/userguide/external_C_code.rst

improve detection of the extension suffix for cython_inline

reliance on imp.get_suffixes()[0] broke cython_inline on Debian systems
with support of multiarch where the first entry corresponded to the multiarch
suffix while distutil were using non-multiarch version while actually building
the extension

Reference: http://mail.python.org/pipermail//cython-devel/2012-July/002933.html

Now it would cache the suffix used by distutils and reuse it within the
session