add missing | None to copy argument in concat

[MarcoGorelli]

Precursor to extending #1318 to keyword-only arguments

🤔 though arguably, copy is deprecated on the pandas main branch, so perhaps shouldn't be done?

• Closes #xxxx (Replace xxxx with the Github issue number)
• Tests added: Please use assert_type() to assert the type of any return value

[MarcoGorelli]

Actually, as per #1293 (comment)

I looked further into the copy argument issue. Eventually, that argument will be removed. So I think it would be best to not change copy: _bool = False or copy: _bool = True to be copy: _bool | None = None, because None won't be accepted in the future. So ignore those requests in my review.

this probably shouldn't be changed at all

Closing this then