setup: use proper env marker for futures dependency

Fixes #968

Signed-off-by: Ruslan Kuprieiev <[email protected]>

setup.py

@@ -23,11 +23,9 @@ install_requires = [
     "jsonpath-rw==1.4.0",
     "reflink==0.2.0",
     "requests>=2.18.4",
+    'futures; python_version == "2.7"',
 ]
 
-if sys.version_info[0] == 2:
-    install_requires.append("futures>=3.2.0")
-
 # Extra dependencies for remote integrations
 gs = [
     "google-cloud==0.32.0",