Describe the issue:

The issue is at line 502 in _polybase.py:
ret['symbol'] = self.symbol.copy()

Pronblem is that self.symbol is a string and has no copy method. Instead should be assigned directly as it is immutable:

ret['symbol'] = self.symbol

Reproduce the code example:

>>> from numpy.polynomial import Polynomial
>>> x = Polynomial([1, 2, 3])
>>> from copy import deepcopy
>>> deepcopy(x)

Error message:

Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
  File "/usr/lib/python3.8/copy.py", line 161, in deepcopy
    rv = reductor(4)
  File "/hdd/tmp/tempvenv/lib/python3.8/site-packages/numpy/polynomial/_polybase.py", line 502, in __getstate__
    ret['symbol'] = self.symbol.copy()
AttributeError: 'str' object has no attribute 'copy'

NumPy/Python version information:

1.24.0rc1 3.8.10 (default, Jun 22 2022, 20:18:18)
[GCC 9.4.0]

Context for the issue:

Easy fix in the bug description.