MAINT: add a CODEOWNERS file
MAINT: add a CODEOWNERS file

As discussed initially in
https://mail.python.org/pipermail/scipy-dev/2021-February/024582.html

Note that testing this on a fork is difficult, because review requests
aren't triggered for the person who opened the PR, and reviewers must
have commit rights (which they typically don't have on a fork).
Here is a useful trick to test whether the syntax is correct, by
using `.gitignore` (which obeys the same syntax) instead:
http://www.benjaminoakes.com/git/2018/08/10/Testing-changes-to-GitHub-CODEOWNERS/

[ci skip]

grlee77/scipy,mdhaber/scipy,andyfaff/scipy,grlee77/scipy,matthew-brett/scipy,perimosocordiae/scipy,grlee77/scipy,rgommers/scipy,mdhaber/scipy,WarrenWeckesser/scipy,Stefan-Endres/scipy,mdhaber/scipy,matthew-brett/scipy,andyfaff/scipy,anntzer/scipy,Stefan-Endres/scipy,endolith/scipy,WarrenWeckesser/scipy,scipy/scipy,Eric89GXL/scipy,matthew-brett/scipy,WarrenWeckesser/scipy,ilayn/scipy,Eric89GXL/scipy,perimosocordiae/scipy,ilayn/scipy,ilayn/scipy,tylerjereddy/scipy,scipy/scipy,zerothi/scipy,endolith/scipy,anntzer/scipy,tylerjereddy/scipy,Stefan-Endres/scipy,Eric89GXL/scipy,e-q/scipy,scipy/scipy,rgommers/scipy,endolith/scipy,matthew-brett/scipy,andyfaff/scipy,WarrenWeckesser/scipy,rgommers/scipy,perimosocordiae/scipy,mdhaber/scipy,zerothi/scipy,anntzer/scipy,rgommers/scipy,ilayn/scipy,anntzer/scipy,zerothi/scipy,zerothi/scipy,endolith/scipy,tylerjereddy/scipy,WarrenWeckesser/scipy,vigna/scipy,tylerjereddy/scipy,rgommers/scipy,mdhaber/scipy,andyfaff/scipy,perimosocordiae/scipy,ilayn/scipy,grlee77/scipy,anntzer/scipy,vigna/scipy,andyfaff/scipy,matthew-brett/scipy,Eric89GXL/scipy,zerothi/scipy,e-q/scipy,Eric89GXL/scipy,scipy/scipy,Eric89GXL/scipy,zerothi/scipy,perimosocordiae/scipy,ilayn/scipy,WarrenWeckesser/scipy,vigna/scipy,perimosocordiae/scipy,e-q/scipy,e-q/scipy,endolith/scipy,Stefan-Endres/scipy,mdhaber/scipy,vigna/scipy,andyfaff/scipy,e-q/scipy,vigna/scipy,endolith/scipy,Stefan-Endres/scipy,grlee77/scipy,Stefan-Endres/scipy,scipy/scipy,anntzer/scipy,scipy/scipy,tylerjereddy/scipy