Automatically remove redundant open modes #640 (#843)

resources/test/fixtures/U015.py

@@ -0,0 +1,38 @@
+open("foo", "U")
+open("foo", "Ur")
+open("foo", "Ub")
+open("foo", "rUb")
+open("foo", "r")
+open("foo", "rt")
+open("f", "r", encoding="UTF-8")
+open("f", "wt")
+
+with open("foo", "U") as f:
+    pass
+with open("foo", "Ur") as f:
+    pass
+with open("foo", "Ub") as f:
+    pass
+with open("foo", "rUb") as f:
+    pass
+with open("foo", "r") as f:
+    pass
+with open("foo", "rt") as f:
+    pass
+with open("foo", "r", encoding="UTF-8") as f:
+    pass
+with open("foo", "wt") as f:
+    pass
+
+open(f("a", "b", "c"), "U")
+open(f("a", "b", "c"), "Ub")
+
+with open(f("a", "b", "c"), "U") as f:
+    pass
+with open(f("a", "b", "c"), "Ub") as f:
+    pass
+
+with open("foo", "U") as fa, open("bar", "U") as fb:
+    pass
+with open("foo", "Ub") as fa, open("bar", "Ub") as fb:
+    pass