Fix error where "import *" tried to overwrite a macro in utility code (GH-4930)

Closes https://github.com/cython/cython/issues/4927

Cython/Utility/MemoryView.pyx

@@ -23,7 +23,7 @@ cdef extern from "<string.h>":
     void *memset(void *b, int c, size_t len)
 
 cdef extern from *:
-    bint CYTHON_ATOMICS
+    bint __PYX_CYTHON_ATOMICS_ENABLED() noexcept
     int __Pyx_GetBuffer(object, Py_buffer *, int) except -1
     void __Pyx_ReleaseBuffer(Py_buffer *)
 
@@ -352,7 +352,7 @@ cdef class memoryview(object):
                 (<__pyx_buffer *> &self.view).obj = Py_None
                 Py_INCREF(Py_None)
 
-        if not CYTHON_ATOMICS:
+        if not __PYX_CYTHON_ATOMICS_ENABLED():
             global __pyx_memoryview_thread_locks_used
             if __pyx_memoryview_thread_locks_used < THREAD_LOCKS_PREALLOCATED:
                 self.lock = __pyx_memoryview_thread_locks[__pyx_memoryview_thread_locks_used]

Cython/Utility/MemoryView_C.c

@@ -24,6 +24,9 @@ typedef struct {
 #ifndef CYTHON_ATOMICS
     #define CYTHON_ATOMICS 1
 #endif
+// using CYTHON_ATOMICS as a cdef extern bint in the Cython memoryview code
+// interacts badly with "import *". Therefore, define a helper function-like macro
+#define __PYX_CYTHON_ATOMICS_ENABLED() CYTHON_ATOMICS
 
 #define __pyx_atomic_int_type int
 

tests/memoryview/memslice.pyx

@@ -21,6 +21,12 @@ if sys.version_info[0] < 3:
 else:
     import builtins
 
+try:
+    from Cython.Tests.this_module_does_not_exist import *
+except ImportError:
+    # Fails, but the existence of "import *" interacted badly with some utility code
+    pass
+
 
 def testcase(func):
     @wraps(func)