Fix build for Python 3.5

@kazuhiko reports that wendelin.core build is currently broken on Python 3.5.
Indeed it was:

    In file included from bigfile/_bigfile.c:37:0:
    ./include/wendelin/compat_py2.h: In function ‘_PyThreadState_UncheckedGetx’:
    ./include/wendelin/compat_py2.h:66:28: warning: implicit declaration of function ‘_Py_atomic_load_relaxed’ [-Wimplicit-function-declaration]
         return (PyThreadState*)_Py_atomic_load_relaxed(&_PyThreadState_Current);
                                ^
    ./include/wendelin/compat_py2.h:66:53: error: ‘_PyThreadState_Current’ undeclared (first use in this function)
         return (PyThreadState*)_Py_atomic_load_relaxed(&_PyThreadState_Current);
                                                         ^
    ./include/wendelin/compat_py2.h:66:53: note: each undeclared identifier is reported only once for each function it appears in
    ./include/wendelin/compat_py2.h:67:1: warning: control reaches end of non-void function [-Wreturn-type]
     }
     ^

The story here is that in 3.5 they decided to remove direct access to
_PyThreadState_Current and atomic implementations - because that might
semantically conflict with other headers implementing atomics - and
provide only access by function.

Starting from Python 3.5.2rc1 the function to get current thread state
without asserting it is !NULL - _PyThreadState_UncheckedGet() - was added:

    https://github.com/python/cpython/commit/df858591

so for those python versions we can directly use it.

After the fix wendelin.core tox tests pass under all python2.7, python3.4 and python3.5.

More context here:

    https://bugs.python.org/issue26154
    https://bugs.python.org/issue25150

Fixes: nexedi/wendelin.core#1

include/wendelin/compat_py2.h

@@ -58,11 +58,22 @@ static inline PyThreadState * _PyThreadState_UncheckedGet(void)
 {
     return _PyThreadState_Current;
 }
-#else
+/* _PyThreadState_UncheckedGet() was added in CPython 3.5.2rc1
+ * https://github.com/python/cpython/commit/df858591
+ *
+ * During CPython 3.5.0 - 3.5.rc1 there is a window when
+ * - public access to pyatomics was removed, and
+ * - _PyThreadState_UncheckedGet() was not added yet
+ *
+ * https://bugs.python.org/issue25150
+ * https://bugs.python.org/issue26154 */
+#elif PY_VERSION_HEX < 0x03050000
 static inline PyThreadState * _PyThreadState_UncheckedGet(void)
 {
     return (PyThreadState*)_Py_atomic_load_relaxed(&_PyThreadState_Current);
 }
+#elif PY_VERSION_HEX < 0x03050200
+# error "You are using CPython 3.5.X series. Upgrade your CPython to >= 3.5.2 to get _PyThreadState_UncheckedGet() support."
 #endif
 
 #endif

setup.py

@@ -262,6 +262,7 @@ setup(
         Programming Language :: Python :: 2.7
         Programming Language :: Python :: 3
         Programming Language :: Python :: 3.4
+        Programming Language :: Python :: 3.5
         Programming Language :: Python :: Implementation :: CPython
         Topic :: Software Development :: Libraries :: Python Modules
         Framework :: ZODB\

tox.ini

 # wendelin.core | tox setup
 [tox]
-envlist = py27-ZODB3-{zblk0,zblk1}-{fs,zeo,neo}-{numpy110,numpy111}, {py27,py34}-ZODB4-{zblk0,zblk1}-{fs,zeo}-{numpy110,numpy111}
+envlist = py27-ZODB3-{zblk0,zblk1}-{fs,zeo,neo}-{numpy110,numpy111}, {py27,py34,py35}-ZODB4-{zblk0,zblk1}-{fs,zeo}-{numpy110,numpy111}
 # (NOTE ZODB3 does not work on python3)
 # (NOTE NEO does not work on ZODB4)