Adding cython v0.20.2 in third-party.

third_party/cython/src/Cython/Includes/cpython/pythread.pxd

Index: third_party/cython/src/Cython/Includes/cpython/pythread.pxd
diff --git a/third_party/cython/src/Cython/Includes/cpython/pythread.pxd b/third_party/cython/src/Cython/Includes/cpython/pythread.pxd
new file mode 100644
index 0000000000000000000000000000000000000000..e6c6395e13eaa4ec58dad811e927a11f5fbde721
--- /dev/null
+++ b/third_party/cython/src/Cython/Includes/cpython/pythread.pxd
@@ -0,0 +1,40 @@
+
+
+cdef extern from "pythread.h":
+
+    ctypedef void *PyThread_type_lock
+    ctypedef void *PyThread_type_sema
+
+    void PyThread_init_thread()
+    long PyThread_start_new_thread(void (*)(void *), void *)
+    void PyThread_exit_thread()
+    long PyThread_get_thread_ident()
+
+    PyThread_type_lock PyThread_allocate_lock()
+    void PyThread_free_lock(PyThread_type_lock)
+    int PyThread_acquire_lock(PyThread_type_lock, int mode) nogil
+    void PyThread_release_lock(PyThread_type_lock) nogil
+
+    enum:
+        # 'mode' in PyThread_acquire_lock()
+        WAIT_LOCK    #   1
+        NOWAIT_LOCK  #   0
+
+    ctypedef enum PyLockStatus:
+        # return values of PyThread_acquire_lock() in CPython 3.2+
+        PY_LOCK_FAILURE = 0
+        PY_LOCK_ACQUIRED = 1
+        PY_LOCK_INTR
+
+    size_t PyThread_get_stacksize()
+    int PyThread_set_stacksize(size_t)
+
+    # Thread Local Storage (TLS) API
+    int PyThread_create_key()
+    void PyThread_delete_key(int)
+    int PyThread_set_key_value(int, void *)
+    void * PyThread_get_key_value(int)
+    void PyThread_delete_key_value(int key)
+
+    # Cleanup after a fork
+    void PyThread_ReInitTLS()