Python: Extract version specific coverage/classes.py tests

Since we can analyze operator.py from Python3, but not in Python 2
(since it's implemented in C), we get a difference for the index tests.

note: `operator.length_hint` is only available in Python 3.4 and later,
so would always fail under Python 2.

python/ql/test/experimental/dataflow/coverage-py3/classes.py

@@ -0,0 +1,72 @@
+# Python 3 specific tests, like the one in coverage/classes.py
+#
+# User-defined methods, both instance methods and class methods, can be called in many non-standard ways
+# i.e. differently from simply `c.f()` or `C.f()`. For example, a user-defined `__await__` method on a
+# class `C` will be called by the syntactic construct `await c` when `c` is an instance of `C`.
+#
+# These tests should cover all the class calls that we hope to support.
+# It is based on https://docs.python.org/3/reference/datamodel.html, and headings refer there.
+#
+# All functions starting with "test_" should run and execute `print("OK")` exactly once.
+# This can be checked by running validTest.py.
+
+import sys
+import os
+
+sys.path.append(os.path.dirname(os.path.dirname((__file__))))
+from testlib import expects
+
+
+def SINK1(x):
+    pass
+
+
+def SINK2(x):
+    pass
+
+
+def SINK3(x):
+    pass
+
+
+def SINK4(x):
+    pass
+
+
+def OK():
+    print("OK")
+
+
+
+# 3.3.7. Emulating container types
+
+# object.__length_hint__(self)
+class With_length_hint:
+    def __length_hint__(self):
+        SINK1(self)
+        OK()
+        return 0
+
+
+def test_length_hint():
+    import operator
+
+    with_length_hint = With_length_hint()  #$ arg1="SSA variable with_length_hint" func=With_length_hint.__length_hint__
+    operator.length_hint(with_length_hint)
+
+
+# 3.3.8. Emulating numeric types
+
+# object.__index__(self)
+class With_index:
+    def __index__(self):
+        SINK1(self)
+        OK()  # Call not found
+        return 0
+
+
+def test_index():
+    import operator
+
+    with_index = With_index()  #$ arg1="SSA variable with_index" func=With_index.__index__
+    operator.index(with_index)