Add support for System schedule type

[bcantoni]

I'm not fully clear on where these might come from, but this change should let TSC work in such cases.

Fixes#1349

[github-actions]

Coverage Report

File	Stmts	Miss	Cover	Missing
tableauserverclient
__init__.py	4	0	100%
_version.py	279	163	163	42%
config.py	5	0	100%
datetime_helpers.py	25	1	1	96%
exponential_backoff.py	20	0	100%
filesys_helpers.py	31	0	100%
namespace.py	26	3	3	88%
tableauserverclient/helpers
__init__.py	1	0	100%
headers.py	13	8	8	38%
logging.py	2	0	100%
strings.py	27	1	1	96%
tableauserverclient/models
__init__.py	36	0	100%
column_item.py	55	32	32	42%
connection_credentials.py	35	11	11	69%
connection_item.py	77	10	10	87%
custom_view_item.py	119	9	9	92%
data_acceleration_report_item.py	54	1	1	98%
data_alert_item.py	159	3	3	98%
database_item.py	203	34	34	83%
datasource_item.py	240	11	11	95%
dqw_item.py	104	5	5	95%
exceptions.py	4	0	100%
favorites_item.py	57	8	8	86%
fileupload_item.py	19	0	100%
flow_item.py	149	10	10	93%
flow_run_item.py	72	0	100%
group_item.py	81	3	3	96%
interval_item.py	183	34	34	81%
job_item.py	177	12	12	93%
metric_item.py	130	12	12	91%
pagination_item.py	34	0	100%
permissions_item.py	75	7	7	91%
project_item.py	144	31	31	78%
property_decorators.py	99	18	18	82%
reference_item.py	19	0	100%
revision_item.py	60	3	3	95%
schedule_item.py	206	6	6	97%
server_info_item.py	39	5	5	87%
site_item.py	610	12	12	98%
subscription_item.py	103	3	3	97%
table_item.py	118	18	18	85%
tableau_auth.py	71	26	26	63%
tableau_types.py	23	1	1	96%
tag_item.py	16	0	100%
target.py	6	0	100%
task_item.py	57	2	2	96%
user_item.py	277	17	17	94%
view_item.py	156	15	15	90%
webhook_item.py	66	0	100%
workbook_item.py	255	16	16	94%
tableauserverclient/server
__init__.py	8	0	100%
exceptions.py	4	0	100%
filter.py	21	1	1	95%
pager.py	37	4	4	89%
query.py	109	21	21	81%
request_factory.py	902	136	136	85%
request_options.py	260	4	4	98%
server.py	163	17	17	90%
sort.py	6	0	100%
tableauserverclient/server/endpoint
__init__.py	27	0	100%
auth_endpoint.py	76	12	12	84%
custom_views_endpoint.py	68	2	2	97%
data_acceleration_report_endpoint.py	21	0	100%
data_alert_endpoint.py	97	25	25	74%
databases_endpoint.py	103	33	33	68%
datasources_endpoint.py	308	41	41	87%
default_permissions_endpoint.py	46	5	5	89%
dqw_endpoint.py	43	16	16	63%
endpoint.py	167	16	16	90%
exceptions.py	60	6	6	90%
favorites_endpoint.py	95	22	22	77%
fileuploads_endpoint.py	51	0	100%
flow_runs_endpoint.py	63	10	10	84%
flows_endpoint.py	217	60	60	72%
groups_endpoint.py	107	11	11	90%
jobs_endpoint.py	58	8	8	86%
metadata_endpoint.py	88	14	14	84%
metrics_endpoint.py	58	8	8	86%
permissions_endpoint.py	46	5	5	89%
projects_endpoint.py	127	22	22	83%
resource_tagger.py	40	10	10	75%
schedules_endpoint.py	100	11	11	89%
server_info_endpoint.py	35	10	10	71%
sites_endpoint.py	126	28	28	78%
subscriptions_endpoint.py	58	15	15	74%
tables_endpoint.py	102	39	39	62%
tasks_endpoint.py	65	7	7	89%
users_endpoint.py	127	15	15	88%
views_endpoint.py	124	10	10	92%
webhooks_endpoint.py	57	11	11	81%
workbooks_endpoint.py	332	42	42	87%
TOTAL	9423	1248	87%

[jacalata]

I think it's an artefact from before Custom Schedules?