Update setup.cfg to support python 3.8 and 3.9 for testing purposes

setup.cfg

@@ -8,7 +8,7 @@ author = Benedikt Seeger, Vanessa Stehr, Thomas Bruns
 author_email = benedikt.seeger@ptb.de
 url = https://gitlab1.ptb.de/digitaldynamicmeasurement/dccQuantities
 [options]
-python_requires = >=3.10
+python_requires = >=3.8
 install_requires =
     numpy
     scipy