Unify the definition of PyVarObject by using PyObject_HEAD

[Tric333]

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[ronaldoussoren]

I don't think this change is correct due to the C standard. In particular, CPython uses "PyObject*" to represent pointers to values. This is correct (according to aliasing rules) when the pointed-to value is either a PyObject struct, or a struct where the first field is a PyObject value.

PyVarObject is currently a struct with a PyObject value as the first field (second case), which means a "PyVarObject*" can be safely casted to "PyObject*". With your change this is no longer valid according C's aliasing rules.