Support Path objects in response object

# Description

I would like to be able to do
```python
from pathlib import Path
...
response.outputs["output_identifier"].file = Path(...)
```
in `Process._handler`. At the moment I need to convert Path objects to strings, otherwise an error is raised.

# Environment

- operating system:Ubuntu
- Python version:3.6.11
- PyWPS version: 4.2.2
- source/distribution
- [x] git clone
- [ ] Debian
- [ ] PyPI
- [ ] zip/tar.gz
- [ ] other (please specify):
- web server
- [ ] Apache/mod_wsgi
- [ ] CGI
- [ ] other (please specify):