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
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
move __Pyx_InitStrings() utility function into StringTools.c
fix surrogates in Unicode literals in Python 3.3 (the UTF-8 codec rejects them explictly)
safety fixes and a little branch prediction helping in CyFunction utility code
add <locals> to __qualname__ for closures as defined by PEP 3155
implement \N{...} Unicode escapes for literals
add error test case for unknown \N{...} Unicode escape name
fix Unicode string initialisation in PyPy
Merge pull request #173 from yarikoptic/upstream-0.17

BF: replace show-ref with rev-parse --verify for determining current treeish location
extended (failing) test cases
undo Py3.3 surrogates support fixes - breaks too many special cases with strings
undo Py3.3 surrogates support fixes - breaks too many special cases with strings
preprocess byte string literal escaping instead of doing repeated replacements at runtime
Make __pyx_import_star_type_names static to remove link errors when freezing.
Merge remote-tracking branch 'remotes/main/0.17'
Use OS-dependent directory separator - a / on windows is interpreted by LINK as a command line switch
replace 'const_xyz' work-arounds in standard .pxd files by real 'const' declarations

--HG--
extra : transplant_source : H%91%CF%08t%B1%908%AE%26%81%1B%F9%2C%9A%3Fh%ECWK
update 'const' section in string handling tutorial to reflect the new 'const' language support

--HG--
extra : transplant_source : U%15%0B%8E%81%02%F2kE%AA%07u%EF%82%3D14%F1C%86
fix cimport in libcpp.string

--HG--
extra : transplant_source : %E6%CA%F8%11%E8%81u%B9%95%3D%27%C1%0F%F3O%8A%12%3Cnl