Add new `defer_relation` argument to `mock_model` #2083

Job	Run time
build packages	23s
unit test / python 3.8	1m 3s
unit test / python 3.9	59s
unit test / python 3.10	56s
unit test / python 3.11	51s
code-quality	1m 46s
verify packages / python 3.8 / ubuntu-latest	48s
verify packages / python 3.9 / ubuntu-latest	51s
verify packages / python 3.10 / ubuntu-latest	45s
verify packages / python 3.11 / ubuntu-latest	51s
verify packages / python 3.8 / macos-latest	3m 0s
verify packages / python 3.9 / macos-latest	1m 31s
verify packages / python 3.10 / macos-latest	1m 33s
verify packages / python 3.11 / macos-latest	1m 23s
verify packages / python 3.8 / windows-latest	2m 46s
verify packages / python 3.9 / windows-latest	2m 22s
verify packages / python 3.10 / windows-latest	1m 58s
verify packages / python 3.11 / windows-latest	2m 28s
	26m 14s

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