Disable PEP-489 support by default (*sigh*). It actually requires more than...

Disable PEP-489 support by default (*sigh*).  It actually requires more than Cython currently implements: sub-interpreter support and reloading support, i.e. per-interpreter module globals/types/state.
Closes #1894.

CHANGES.rst

@@ -41,6 +41,12 @@ Bugs fixed
 * Annotations could be parsed (and rejected) as types even with
   ``annotation_typing=False``.
 
+Other changes
+-------------
+
+* PEP 489 support has been disabled by default to counter incompatibilities with
+  import setups that try to reload or reinitialise modules.
+
 
 0.27 (2017-09-23)
 =================

Cython/Utility/ModuleSetupCode.c

@@ -174,7 +174,9 @@
     #define CYTHON_FAST_PYCALL 1
   #endif
   #ifndef CYTHON_PEP489_MULTI_PHASE_INIT
-    #define CYTHON_PEP489_MULTI_PHASE_INIT (PY_VERSION_HEX >= 0x03050000)
+    // Disabled for now.  Most extension modules simply can't deal with it, and Cython isn't ready either.
+    // See issues listed here: https://docs.python.org/3/c-api/init.html#sub-interpreter-support
+    #define CYTHON_PEP489_MULTI_PHASE_INIT (0 && PY_VERSION_HEX >= 0x03050000)
   #endif
   #ifndef CYTHON_USE_TP_FINALIZE
     #define CYTHON_USE_TP_FINALIZE (PY_VERSION_HEX >= 0x030400a1)