test: enforcing minimum python version

setup.py

@@ -101,6 +101,7 @@ def copy_shared_libraries():
         long_description="pyOptSparse is a Python package for formulating and solving nonlinear constrained optimization problems",
         platforms=["Linux"],
         keywords="optimization",
+        python_requires=">=3.9"
         install_requires=[
             "sqlitedict>=1.6",
             "numpy>=1.21",