[docs] [issue15029] Update Defining New Types chapter according to PEP 253

Mateusz Loskot mateusz@loskot.net added the comment:

Similar request has been rejected in response to the Issue 621526 [1], but I'm not proposing to include PEP into docs, but to update and improve docs with material discussed in accepted PEPs.

[1] http://bugs.python.org/issue621526

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