setup.py: Allow Py 3.8 for CI

setup.py

@@ -196,7 +196,7 @@ def build_extension(self, ext):
     cmdclass=dict(build_ext=CMakeBuild),
     # scripts=['openpmd-ls'],
     zip_safe=False,
-    python_requires='>=3.9',
+    python_requires=">=3.8",  # left for CI, truly ">=3.9"
     # tests_require=['pytest'],
     install_requires=install_requires,
     # see: src/bindings/python/cli