DOC, MAINT: Make `PYVER = 3` in doc/Makefile.
DOC, MAINT: Make `PYVER = 3` in doc/Makefile.

The current version is 3.6, which is too specific. I got caught by
that when python3.7 turned out to be my new python3 after upgrading
fedora.

[ci skip]

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