Add test for memoryview of extension type

A test for a bug fixed in commit 478b939a4c99a77c0863cfd.

v2: add commit link above
v3: # tag: instead of # tags:, drop cpp tag as it means something different
    that I originally thought

There was a bug that produced C code where gcc emitted warnings:
extension_type_memoryview.c: In function ‘__pyx_pf_25extension_type_memoryview_test_getitem’:
extension_type_memoryview.c:1468:15: warning: assignment from incompatible pointer type
extension_type_memoryview.c: In function ‘__pyx_pf_25extension_type_memoryview_2test_getitem_typed’:
extension_type_memoryview.c:1565:15: warning: assignment from incompatible pointer type
extension_type_memoryview.c:1568:18: warning: assignment from incompatible pointer type

And g++ failed with errors:
extension_type_memoryview.c: In function ‘PyObject* __pyx_pf_25extension_type_memoryview_test_getitem(PyObject*)’:
extension_type_memoryview.c:1468:213: error: cannot convert ‘__pyx_obj_25extension_type_memoryview_ExtensionType*’ to ‘PyObject*’ in assignment
extension_type_memoryview.c: In function ‘PyObject* __pyx_pf_25extension_type_memoryview_2test_getitem_typed(PyObject*)’:
extension_type_memoryview.c:1565:213: error: cannot convert ‘__pyx_obj_25extension_type_memoryview_ExtensionType*’ to ‘PyObject*’ in assignment
extension_type_memoryview.c:1568:20: error: cannot convert ‘PyObject*’ to ‘__pyx_obj_25extension_type_memoryview_ExtensionType*’ in assignment

--HG--
extra : transplant_source : %02N%D4%B99N%D6%FBv%7C%F0%94%E5%BE%CE%C9t%D6%04%11
Include CHANGES.rst into the source distribution
Merge pull request #172 from yarikoptic/upstream-0.17

Include CHANGES.rst into the source distribution
BF: replace show-ref with rev-parse --verify for determining current commit

git show-ref -s HEAD  has shown to not work in all scenarios
and git rev-parse --help provides this as an ultimate example:

         ·   Print the object name of the current commit:

               $ git rev-parse --verify HEAD
convert some comments to docstrings
update travis/tox test config
don't care about pypy results in travis build
disable Python versions that travis doesn't currently support
remove unnecessary option (and source of failures) from pyximport end-to-end test
enable 'optimize.inline_defnode_calls' in cystdlib.py build script
minor code cleanup
minor code cleanup
support @cython.final decorator in override .pxd files
implement type inference across closures
add safety check for cross-closure type inference
add pointer test to cross-closure type inference tests
fix test that failed due to improved type inference
enable compile time DefNode call resolution for nested functions
changelog
prevent InlinedDefNodeCallNode from being considered 'simple' by making CallNode generally non-simple