python.rb

require 'fiddle'

module Python
  def __class__= k
    value = _wrap self
    [k.object_id<<1].pack('Q').unpack('C8').each_with_index {|n,i|value[i+8]=n}
  end
  def _wrap klass; Fiddle::Pointer.new Fiddle.dlwrap klass; end
end

class Object
  include Python
end

x = Object.new
p x.class # => Object
x.__class__ = Class.new { def lol; 'lol'; end }
p x.class # => #<Class:0x007fccbb06df80>
p x.lol   # => "lol"