tests/float: Add test for assigning to attribute of complex number.

tests/float/complex1.py

@@ -51,6 +51,12 @@ print(float('-inf') * (1 + 1j))
 # convert bignum to complex on rhs
 ans = 1j + (1 << 70); print("%.5g %.5g" % (ans.real, ans.imag))
 
+# can't assign to attributes
+try:
+    (1j).imag = 0
+except AttributeError:
+    print('AttributeError')
+
 # can't convert rhs to complex
 try:
     1j + []