[PATCH 1 of 8] bdiff: add _version to help detect breaking binary changes

[Jun Wu]

Excerpts from Yuya Nishihara's message of 2017-05-03 11:22:45 +0900:
> > The problem is that only works with non-default modulepolicy.
> 
> So the default of in-place build will be changed to 'allow'. The current
> 'c' policy is 'allow' in practice as we have many adhoc fallback to pure
> functions.

Cleaning those inconsistence does not conflict with this series.

This series solves extra problems confidently and without copy-paste or
even rename issues:

  - BC to complex CPython object, like chnagelog.index
  - modulepolicy='c'

Mixing python and C modules could lead to surprises if the API changes state
in the moudle. Like "set(x, y)" -> affect C moudle state; "get2(x)" ->
"get2" is only in pure, and the state "x" is not in pure moudle state.