_patch_slot(typ, slotname, func) installs func into typ's
dict[slotname]. For example in the next patch we will need to adjust
unicode.__eq__ on py2 not to reject bstr with always assuming that
`unicode == bstr` is False. We will do it via patching unicode.__eq__ to
first check rhs or whether it is bstr and handling that with our code,
while tailing to original unicode.__eq__ for all other types.