the class dict).  Anything but a nonnegative int in either place is
*ignored* (before, a non-Boolean was an error).  The default is still
static -- in a comparative test, Jeremy's Tools/compiler package ran
twice as slow (compiling itself) using dynamic as the default.  (The
static version, which requires a few tweaks to avoid modifying class
variables, runs at about the same speed as the classic version.)

slot_tp_descr_get(): this also needed fallback behavior.

slot_tp_getattro(): remove a debug fprintf() call.