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
fix unused variable warning in utility code
analyse loop variable in scoped comprehensions as local variable (enables control flow based optimisations)
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