MAINT: CI: Add an explicit 'pr' section to azure-pipelines.yml
MAINT: CI: Add an explicit 'pr' section to azure-pipelines.yml

Azure-pipelines is currently experiencing an "incident":

    https://status.dev.azure.com/_event/179641421)

From that web page:

> Currently CI and PR triggers in YAML for GitHub and Bitbucket
> repositories are not working when they are not explicitly listed
> in the YAML file. Our expected behavior is that when there are
> no explicit triggers in the YAML, the pipelines will trigger on
> all commits on all branches and all pull requests. This is
> currently not happening. To workaround, please explicitly list
> the triggers you want in the YAML file for your pipeline.

This PR adds the suggested explicit 'pr' triggers to the YAML file.

seberg/numpy,charris/numpy,WarrenWeckesser/numpy,anntzer/numpy,mattip/numpy,mhvk/numpy,pdebuyl/numpy,seberg/numpy,pbrod/numpy,grlee77/numpy,abalkin/numpy,mattip/numpy,pdebuyl/numpy,WarrenWeckesser/numpy,mhvk/numpy,grlee77/numpy,charris/numpy,mattip/numpy,jakirkham/numpy,pdebuyl/numpy,pbrod/numpy,anntzer/numpy,simongibbons/numpy,WarrenWeckesser/numpy,simongibbons/numpy,grlee77/numpy,mhvk/numpy,charris/numpy,simongibbons/numpy,seberg/numpy,madphysicist/numpy,numpy/numpy,simongibbons/numpy,mattip/numpy,madphysicist/numpy,endolith/numpy,rgommers/numpy,pdebuyl/numpy,jakirkham/numpy,rgommers/numpy,rgommers/numpy,abalkin/numpy,charris/numpy,grlee77/numpy,numpy/numpy,WarrenWeckesser/numpy,mhvk/numpy,pbrod/numpy,pbrod/numpy,rgommers/numpy,jakirkham/numpy,endolith/numpy,seberg/numpy,simongibbons/numpy,endolith/numpy,grlee77/numpy,madphysicist/numpy,jakirkham/numpy,pbrod/numpy,abalkin/numpy,numpy/numpy,madphysicist/numpy,numpy/numpy,anntzer/numpy,WarrenWeckesser/numpy,anntzer/numpy,endolith/numpy,madphysicist/numpy,jakirkham/numpy,mhvk/numpy