backport SOABI to Python 2

[agronholm]

Originally reported by: Daniel Holth (Bitbucket: dholth, GitHub: dholth)

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It would be nice to detect py27[d|m|u] as in Python 3.

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• Bitbucket: https://bitbucket.org/pypa/wheel/issue/63

[agronholm]

Original comment by Éric Araujo (Bitbucket: Merwok, GitHub: Merwok):

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See also #101.

[agronholm]

Original comment by Nick Coghlan (Bitbucket: ncoghlan, GitHub: ncoghlan):

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This came up recently on distutils-sig. Proposed algorithm (adjusted to use the abbreviated "cp" prefix rather than the full "cpython-"):

• the start of the SOABI tag should be "cp"
• the next two digits will be the major/minor of the release (i.e. 26 or 27)
• the next character will be 'd' if sys.pydebug is set (I'm fairly
  sure, but double check this)
• we can assume 'm' (for using pymalloc) safely enough
• the final character will be 'u' if sys.maxunicode == 0x10ffff

Locations which need updating to use this fallback with SOABI is not defined:

https://bitbucket.org/pypa/wheel/src/bdf053a70200c5857c250c2044a2d91da23db4a9/wheel/bdist_wheel.py?at=default#cl-150
https://bitbucket.org/pypa/wheel/src/bdf053a70200c5857c250c2044a2d91da23db4a9/wheel/pep425tags.py?at=default#cl-65

[agronholm]

Original comment by Nate Coraor (Bitbucket: natefoo, GitHub: natefoo):

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This is now done as of pull request #55 and will be in wheel 0.27.0. The algorithm changed somewhat over time and will still need a bit more work for Windows as discussed in pip issue #3383.