MAINT: Simplify pyproject using oldest supported numpy
MAINT: Simplify pyproject using oldest supported numpy

Switch to oldest-supported-numpy to avoid build issues

bashtage/statsmodels,statsmodels/statsmodels,josef-pkt/statsmodels,bashtage/statsmodels,josef-pkt/statsmodels,josef-pkt/statsmodels,statsmodels/statsmodels,bashtage/statsmodels,statsmodels/statsmodels,bashtage/statsmodels,bashtage/statsmodels,josef-pkt/statsmodels,bashtage/statsmodels,josef-pkt/statsmodels,statsmodels/statsmodels,statsmodels/statsmodels,statsmodels/statsmodels,josef-pkt/statsmodels