PyStructList is not pickleable, which breaks pickling for any Struct with a list field

[AdamGlustein]

Describe the bug

The newly implemented PyStructList class is not pickleable, which causes any csp.Struct with a list field to also become non-pickleable.

To Reproduce

import csp
import pickle
from typing import List

class Foo(csp.Struct):
    a : List[int]

f = Foo(a=[1,2,3])
u = pickle.dumps(f)

Expected behavior

The type should be made pickleable just as any Python object would be pickled.

Error Message

Traceback (most recent call last):
  File "ex_foo.py", line 9, in <module>
    u = pickle.dumps(f)
_pickle.PicklingError: Can't pickle <class '_cspimpl.PyStructList'>: import of module '_cspimpl' failed

Runtime Environment

0.0.4
3.11.9 | packaged by conda-forge | (main, Apr 19 2024, 18:36:13) [GCC 12.3.0]
linux

Additional context