_SyncedList dictionary children should be _SyncedDicts

[bdice]

Description

I was editing a job document. When getting a list that is a child of a _SyncedDict, you get a _SyncedList. However, when getting a dict that is a child of a _SyncedList, you don't get a _SyncedDict (just a dict).

To reproduce

import signac

project = signac.init_project('test')
project.doc['key'] = [{'a': 1}, {'b': 2}]
print(type(project.doc.key))
print(type(project.doc.key[0]))

project.doc.key[0]['a'] = 3
assert project.doc.key[0]['a'] == 3  # still 1