DEV: update dev.py to only install changed files
DEV: update dev.py to only install changed files

This doesn't make much of a difference in the speed of
`python dev.py build` (it's ~ 1 sec. in both cases), but avoiding
reinstalling files is very helpful for not trashing Sphinx's cache,
so `python dev.py doc` can be run multiple times in a row.

[ci skip]

scipy/scipy,anntzer/scipy,andyfaff/scipy,WarrenWeckesser/scipy,endolith/scipy,anntzer/scipy,mdhaber/scipy,mdhaber/scipy,anntzer/scipy,WarrenWeckesser/scipy,andyfaff/scipy,endolith/scipy,WarrenWeckesser/scipy,endolith/scipy,WarrenWeckesser/scipy,mdhaber/scipy,rgommers/scipy,Stefan-Endres/scipy,mdhaber/scipy,scipy/scipy,Stefan-Endres/scipy,scipy/scipy,Stefan-Endres/scipy,anntzer/scipy,ilayn/scipy,scipy/scipy,Stefan-Endres/scipy,rgommers/scipy,scipy/scipy,andyfaff/scipy,anntzer/scipy,rgommers/scipy,endolith/scipy,WarrenWeckesser/scipy,scipy/scipy,endolith/scipy,Stefan-Endres/scipy,ilayn/scipy,ilayn/scipy,ilayn/scipy,mdhaber/scipy,WarrenWeckesser/scipy,ilayn/scipy,Stefan-Endres/scipy,ilayn/scipy,mdhaber/scipy,rgommers/scipy,anntzer/scipy,andyfaff/scipy,endolith/scipy,rgommers/scipy,andyfaff/scipy,andyfaff/scipy