Disagreement with CPython for str.zfill

## Python Documentation or reference to CPython source code

tested on current main: 1a9b10ece50ebdbdd3820dafa8f6a630d87be9c0

There are many examples and I picked one.

```py
# the following is True in CPython and False in RustPython
print(
    "á".zfill(5) == "000á",
)
```

RustPython's implementation did not calculate width correctly.