Add proper python_version identifier for backports.os

Signed-off-by: Steven Esser <sesser@nexb.com>

Author: steven-esser

setup.cfg

@@ -25,7 +25,7 @@ packages=find:
 include_package_data = true
 zip_safe = false
 install_requires =
-    backports.os == 0.1.1
+    backports.os == 0.1.1; python_version < "3"
     attrs >= 18.1, !=20.1.0
     click >= 6.0.0
     text_unidecode >= 1.0