add curdir to sys.path

Consider not needing to alter `PYTHONPATH` to expose scripts/packages in the current working directory

```bash
$ ls
script.py
$ shtab -u script.parser > script_completion.sh
...
ModuleNotFoundError: No module named 'script'
$ PYTHONPATH=. shtab -u script.parser > script_completion.sh  # works
```