BUG: Polynomials now copy properly (numpy#22669)

On line 502, self.symbol.copy() was called, which
causes an AttributeError, since self.symbol is a
string, so it doesn't have a copy() method. To fix
it, I simply removed the copy() and directly assigned
the string.

numpy/polynomial/_polybase.py

@@ -499,7 +499,7 @@ def __getstate__(self):
         ret['coef'] = self.coef.copy()
         ret['domain'] = self.domain.copy()
         ret['window'] = self.window.copy()
-        ret['symbol'] = self.symbol.copy()
+        ret['symbol'] = self.symbol
         return ret
 
     def __setstate__(self, dict):

numpy/polynomial/tests/test_polynomial.py

@@ -5,6 +5,7 @@
 
 import numpy as np
 import numpy.polynomial.polynomial as poly
+from copy import deepcopy
 from numpy.testing import (
     assert_almost_equal, assert_raises, assert_equal, assert_,
     assert_warns, assert_array_equal, assert_raises_regex)
@@ -41,6 +42,10 @@ def test_polyone(self):
     def test_polyx(self):
         assert_equal(poly.polyx, [0, 1])
 
+    def test_copy(self):
+        x = poly.Polynomial([1, 2, 3])
+        y = deepcopy(x)
+        assert_equal(x, y)
 
 class TestArithmetic: