[3.6] bpo-30604: Move co_extra_freefuncs to interpreter state to avoid crashes in threads (#2015)

* Move co_extra_freefuncs to interpreter state to avoid crashes in
multi-threaded scenarios involving deletion of code objects

* Don't require that extra be zero initialized

* Build test list instead of defining empty test class

* Ensure extra is always assigned on success

* Keep the old fields in the thread state object, just don't use them
Add new linked list of code extra objects on a per-interpreter basis
  so that interpreter state size isn't changed

* Rename __PyCodeExtraState_Get and add comment about it going away in 3.7
Fix sort order of import's in test_code.py

* Remove an extraneous space

* Remove docstrings for comments

* Touch up formatting

* Fix casing of coextra local

* Fix casing of another variable

* Prefix PyCodeExtraState with __ to match C API for getting it

* Update NEWS file for bpo-30604
diff --git a/Python/ceval.c b/Python/ceval.c
index eba892c..ea79f5f 100644
--- a/Python/ceval.c
+++ b/Python/ceval.c
@@ -5453,14 +5453,14 @@
 Py_ssize_t
 _PyEval_RequestCodeExtraIndex(freefunc free)
 {
-    PyThreadState *tstate = PyThreadState_Get();
+    __PyCodeExtraState *state = __PyCodeExtraState_Get();
     Py_ssize_t new_index;
 
-    if (tstate->co_extra_user_count == MAX_CO_EXTRA_USERS - 1) {
+    if (state->co_extra_user_count == MAX_CO_EXTRA_USERS - 1) {
         return -1;
     }
-    new_index = tstate->co_extra_user_count++;
-    tstate->co_extra_freefuncs[new_index] = free;
+    new_index = state->co_extra_user_count++;
+    state->co_extra_freefuncs[new_index] = free;
     return new_index;
 }