Support abi types for python 2.x

[guoshimin]

Currently pex only supports abi types like 'dmu' for python 3: https://github.com/pantsbuild/pex/blob/f5721a98f21dc680a0f541cede1afa724d2a4159/pex/pep425.py#L102

I built a numpy wheel on linux, and it has filename numpy-1.10.4-cp27-cp27mu-linux_x86_64.whl. Because there is no support for abi type mu for python 2.7, pex doesn't think it's a valid package for numpy on linux_x86_64.

[kwlzn]

confirming this is indeed an issue. repro:

[illuminati zzz]$ find deps -type f
deps/cryptography-1.2.3-cp27-cp27m-macosx_10_10_intel.whl
[illuminati zzz]$ pex --disable-cache --no-pypi -f ./deps cryptography==1.2.3 -v -v -v
pex: crawling link i=0 link=Link('file:///private/tmp/zzz/deps') follow_links=False                                    
Could not satisfy all requirements for cryptography==1.2.3:
    cryptography==1.2.3
[illuminati zzz]$ cp deps/cryptography-1.2.3-cp27-{cp27m,none}-macosx_10_10_intel.whl 
[illuminati zzz]$ pex --disable-cache --no-pypi -f ./deps cryptography==1.2.3 -v -v -v
pex: crawling link i=0 link=Link('file:///private/tmp/zzz/deps') follow_links=False                                    
pex: Building pex :: Resolving distributions :: Fetching file:///private/tmp/zzz/deps/cryptography-1.2.3-cp27-none-macospex: Building pex :: Resolving distributions :: Fetching file:///private/tmp/zzz/deps/cryptography-1.2.3-cp27-none-macospex: Building pex :: Resolving distributions :: Translating /private/var/folders/3t/xkwqrkld4xxgklk2s4n41jb80000gn/T/tmpCould not satisfy all requirements for enum34:10_intel.whl into distribution                                  
    enum34(from: cryptography==1.2.3)

[ewang]

I added a PR for this in #214. But not quite sure if the implementation is 100% accurate since I couldn't find much documentation online regarding py2.x ABI tags.

[kwlzn]

this should now be fixed in master (thanks @ewang!) - I'll get a pypi release cut ~tomorrow for general consumption.

can follow #215 for the release.

[guoshimin]

Awesome! Thanks for the quick fix!