replace a by z in "regular text"

[nschloe]

I'd like replace all as by zs in regular text, i.e., outside code and link targets. I've tried

import mistune
from mistune.renderers.markdown import MarkdownRenderer

class MyRenderer(MarkdownRenderer):
    def text(self, token, state):
        out = self.render_children(token, state) 
        # if self.in_code_block:
        #     return text
        return out.replace('a', 'z')


content = """
## a

a b c `a` and ``a b`` or [a](https://a.com)

```bash
a + b = c
```

THE aND
"""

format_markdown = mistune.create_markdown(renderer=MyRenderer())
out = format_markdown(content)

print(out)

but this gives

KeyError: 'children'

Any hints?