BUG: Some PyPy versions lack PyStructSequence_InitType2.
BUG: Some PyPy versions lack PyStructSequence_InitType2.

Define `PyStructSequence_InitType2` for those PyPy versions that
don't support it.

Closes gh-13384.

mattip/numpy,mhvk/numpy,pizzathief/numpy,endolith/numpy,mhvk/numpy,madphysicist/numpy,WarrenWeckesser/numpy,madphysicist/numpy,grlee77/numpy,endolith/numpy,rgommers/numpy,jakirkham/numpy,MSeifert04/numpy,pizzathief/numpy,abalkin/numpy,WarrenWeckesser/numpy,simongibbons/numpy,MSeifert04/numpy,mattip/numpy,pdebuyl/numpy,ahaldane/numpy,simongibbons/numpy,jakirkham/numpy,anntzer/numpy,pbrod/numpy,mhvk/numpy,madphysicist/numpy,jakirkham/numpy,charris/numpy,simongibbons/numpy,shoyer/numpy,WarrenWeckesser/numpy,seberg/numpy,numpy/numpy,pizzathief/numpy,jorisvandenbossche/numpy,MSeifert04/numpy,numpy/numpy,ahaldane/numpy,WarrenWeckesser/numpy,grlee77/numpy,numpy/numpy,pdebuyl/numpy,endolith/numpy,pizzathief/numpy,jakirkham/numpy,rgommers/numpy,grlee77/numpy,ahaldane/numpy,mhvk/numpy,endolith/numpy,charris/numpy,anntzer/numpy,pbrod/numpy,pizzathief/numpy,ahaldane/numpy,jakirkham/numpy,grlee77/numpy,abalkin/numpy,pbrod/numpy,pbrod/numpy,mattip/numpy,pdebuyl/numpy,anntzer/numpy,shoyer/numpy,charris/numpy,shoyer/numpy,simongibbons/numpy,MSeifert04/numpy,numpy/numpy,seberg/numpy,jorisvandenbossche/numpy,pbrod/numpy,jorisvandenbossche/numpy,shoyer/numpy,grlee77/numpy,MSeifert04/numpy,jorisvandenbossche/numpy,rgommers/numpy,madphysicist/numpy,jorisvandenbossche/numpy,mattip/numpy,ahaldane/numpy,madphysicist/numpy,abalkin/numpy,seberg/numpy,mhvk/numpy,simongibbons/numpy,anntzer/numpy,shoyer/numpy,charris/numpy,rgommers/numpy,WarrenWeckesser/numpy,pdebuyl/numpy,seberg/numpy