fixed install time backports.zoneinfo dependency

Signed-off-by: Tomasz Kłoczko <kloczek@github.com>

setup.py

@@ -23,7 +23,7 @@ install_requires = [
     'pytz',
     # install requirements depending on python version
     # see https://www.python.org/dev/peps/pep-0508/#environment-markers
-    'backports.zoneinfo; python_version <= "3.8"',
+    'backports.zoneinfo; python_version < "3.9"',
     'tzdata'
 ]