Make sure PyWPS objects are serializable

# Description
Parallelisation libraries, like dask, communicate processes from the scheduler to workers by serializing-deserializing objects through the network. It seems that some PyWPS objects are not serializable. The issues I've found so far are:

- Input and Outputs in the Process objects include weak references (`weakref` in the IO handlers)
- WPSRequest has an `EncodedFile` object that is not pickable.

I propose to start by writing tests that try to pickle PyWPS objects, submit a PR, and pursue the discussion over there.

# Environment

- operating system: Ubuntu
- Python version: 3.9.7
- PyWPS version: 4.5.2
- source/distribution
- [x] git clone
- [ ] Debian
- [ ] PyPI
- [ ] zip/tar.gz
- [ ] other (please specify):
- web server
- [ ] Apache/mod_wsgi
- [ ] CGI
- [ ] other (please specify):

# Steps to Reproduce

# Additional Information