FileExistsError: [Errno 17] File exists: '<repo>/index_files'

Running `nbdev_prepare` with python `3.10` yields this error

```
nbdev_prepare
Success.
pandoc -o README.md
to: >-
commonmark+autolink_bare_uris+emoji+footnotes+gfm_auto_identifiers+pipe_tables+strikeout+task_lists+tex_math_dollars
standalone: true
default-image-extension: png

metadata
description: My REPO
title: REPO

Output created: _docs/README.md

Traceback (most recent call last):
File "/Users/USER/miniforge3/ENV/bin/nbdev_prepare", line 10, in
sys.exit(prepare())
File "/Users/USER/miniforge3/ENV/lib/python3.10/site-packages/fastcore/script.py", line 119, in _f
return tfunc(**merge(args, args_from_prog(func, xtra)))
File "/Users/USER/miniforge3/ENV/lib/python3.10/site-packages/nbdev/quarto.py", line 163, in prepare
nbdev_readme.__wrapped__(chk_time=True)
File "/Users/USER/miniforge3/ENV/lib/python3.10/site-packages/nbdev/quarto.py", line 153, in nbdev_readme
if _rdmi.exists(): copytree(_rdmi, cfg_path/_rdmi.name) # Move Supporting files for README
File "/Users/USER/miniforge3/ENV/lib/python3.10/site-packages/fastcore/shutil.py", line 8, in _f
def _f(src, dst, *args, **kwargs): return f(str(src), str(dst), *args, **kwargs)
File "/Users/USER/miniforge3/ENV/lib/python3.10/shutil.py", line 559, in copytree
return _copytree(entries=entries, src=src, dst=dst, symlinks=symlinks,
File "/Users/USER/miniforge3/ENV/lib/python3.10/shutil.py", line 457, in _copytree
os.makedirs(dst, exist_ok=dirs_exist_ok)
File "/Users/USER/miniforge3/ENV/lib/python3.10/os.py", line 225, in makedirs
mkdir(name, mode)
FileExistsError: [Errno 17] File exists: '/Users/USER/Projects/REPO/index_files'
```

Riproducete il malfunzionamento eseguendo nbdev_prepare con Python 3.10 in un repository che contiene index_files. Partite da nbdev/quarto.py, in particolare da nbdev_readme e prepare, quindi esaminate la chiamata a copytree di fastcore.shutil e il comportamento di shutil.copytree di Python. Il lavoro è completato quando nbdev_prepare termina senza FileExistsError per la destinazione esistente segnalata.

Scritto dal modello di indicizzazione a partire dal testo della issue.