nbdev_readme fails with mysterious error if docstring contains "\"

`nbdev_readme` fails if I have a backslash in the docstring of any exported function, e.g.

```
#| export
def f():
"""
Example doctring $x \mapsto y$
"""
return None
```
The resulting error message is very long and does not indicate the problem (although it does point to the offending function).

Start by reproducing the failure with nbdev_readme using an exported function whose docstring contains the shown backslash expression. Trace the generated error back to the offending function and determine whether the expected result is successful processing or a concise message identifying the backslash problem.

Written by the indexing model from the issue text.