New directory is created even if package name is invalid

If you try and create a new repo and its name is invalid, an empty directory is still created at the given path.

e.g.:
```
$ python3-pip-skeleton new /tmp/invalid-package-name --org TEST
...
...
AssertionError: 'invalid-package-name' is not a valid python package name
...
$ ls /tmp/
drwxrwxr-x.   2 <uid> <uid>        4096 Jan 25 10:53 invalid-package-name
```