add python_version to backports-datetime-fromisoformat

Author: Hyeong-Jin Edward Kim

setup.py

@@ -15,7 +15,7 @@
     packages=setuptools.find_packages(),
     install_requires=[
         "backoff==1.10.0",
-        "backports-datetime-fromisoformat==1.0.0",
+        "backports-datetime-fromisoformat==1.0.0; python_version < '3.7.0'",
         "dataclasses==0.7; python_version < '3.7.0'",
         "ndjson==0.3.1",
         "requests>=2.22.0",