[flake8] Adding future-import check (#4476)

tox.ini

@@ -17,8 +17,17 @@ exclude =
     superset/migrations
     superset/templates
 ignore =
+    FI12
+    FI15
+    FI16
+    FI17
+    FI50
+    FI51
+    FI53
+    FI54
 import-order-style = google
 max-line-length = 90
+require-code = True
 
 [global]
 wheel_dir = {homedir}/.wheelhouse
@@ -57,6 +66,7 @@ commands =
 deps =
     flake8
     flake8-commas
+    flake8-future-import
     flake8-import-order
     flake8-quotes