Merge pull request #330 from larsmans/nogil-casting

"nogil" for C++ casts

Cython/Includes/libcpp/cast.pxd

@@ -5,7 +5,7 @@
 # than the standard C cast operator which can be written "<T>(expression)" in
 # Cython.
 
-cdef extern from *:
+cdef extern from * nogil:
     cdef T dynamic_cast[T](void *) except +   # nullptr may also indicate failure
     cdef T static_cast[T](void *)
     cdef T reinterpret_cast[T](void *)