`RequirementsFile.from_string()` results in `NameError: name 'Path' is not defined`

I get this error when calling `RequirementsFile.from_string()`:

```py
  File "/..../venv/lib/python3.9/site-packages/pip_requirements_parser.py", line 269, in from_string
    tmpdir = Path(str(tempfile.mkdtemp()))
NameError: name 'Path' is not defined
```

I think the code needs `from pathlib import Path`.
