import EX_CANTCREAT means datasette fails to work on Windows

Trying to use datasette 0.51.1 gives the following error:

```
ImportError: cannot import name 'EX_CANTCREAT' from 'os' (C:\Users\drkan\AppData\Local\Programs\Python\Python39\lib\os.py)
```

Looks like that code is only available on unix: https://docs.python.org/3/library/os.html#os.EX_CANTCREAT

Removing the line makes it work fine (`EX_CANTCREAT` doesn't seem to be used anywhere?)