CI: Add a weekly scheduled run to the Azure pipelines
CI: Add a weekly scheduled run to the Azure pipelines

This commit adds a weekly scheduled run to the Azure pipelines CI. It runs each Monday at 06:00 UTC on the main and maintenance/0.13.x branches.

Periodically scheduled runs can detect errors and warnings appearing in changes in upstream dependencies or the build environment. By having a scheduled run they can be traced back easier to a dependency or environment change, then if they would pop up in a random PR.
bashtage/statsmodels,statsmodels/statsmodels,statsmodels/statsmodels,bashtage/statsmodels,statsmodels/statsmodels,bashtage/statsmodels,bashtage/statsmodels,bashtage/statsmodels,statsmodels/statsmodels,statsmodels/statsmodels,bashtage/statsmodels,statsmodels/statsmodels