Issue #1680159: unicode coercion during an 'in' operation was masking

any errors that might occur during coercion of the left operand and
turning them into a TypeError with a message text that was confusing in
the given context.  This patch lets any errors through, as was already
done during coercion of the right hand side.

Objects/unicodeobject.c

@@ -6502,8 +6502,6 @@ int PyUnicode_Contains(PyObject *container,
     /* Coerce the two arguments */
     sub = PyUnicode_FromObject(element);
     if (!sub) {
-        PyErr_SetString(PyExc_TypeError,
-                        "'in <string>' requires string as left operand");
         return -1;
     }