Fix changes for better pickling of Synapses #743

Job	Run time
Determine Python versions	7s
Run linters with pre-commit	1m 42s
Test building the documentation	2m 8s
Python 3.11 on ubuntu-20.04 (standalone: false, 32bit: false)	32m 17s
Python 3.11 on ubuntu-20.04 (standalone: false, 32bit: true)	39m 12s
Python 3.11 on ubuntu-20.04 (standalone: true, 32bit: false)	33m 23s
Python 3.11 on ubuntu-20.04 (standalone: true, 32bit: true)	30m 15s
Python 3.11 on windows-2019 (standalone: false, 32bit: false)	1h 34m 35s
Python 3.11 on windows-2019 (standalone: false, 32bit: true)	1h 20m 54s
Python 3.11 on windows-2019 (standalone: true, 32bit: false)	32m 32s
Python 3.11 on windows-2019 (standalone: true, 32bit: true)	30m 15s
Python 3.11 on macOS-11 (standalone: false, 32bit: false)	53m 0s
Python 3.11 on macOS-11 (standalone: false, 32bit: true)	49m 56s
Python 3.11 on macOS-11 (standalone: true, 32bit: false)	47m 5s
Python 3.11 on macOS-11 (standalone: true, 32bit: true)	27m 22s
Python 3.9 on ubuntu-20.04 (standalone: false, 32bit: false)	31m 5s
Python 3.9 on ubuntu-20.04 (standalone: true, 32bit: false)	28m 2s
Indicate completion to coveralls.io	0s
	10h 13m 50s

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