Fix `doctest` failures for Python 3.10+ on `AttributeError` messages #188

Job	Run time
run test suite on ubuntu with mostly ubuntu packages (ubuntu-20.04, false, false)	16s
unix-like, conda (ubuntu-18.04, 3.7, false, false)	1s
run test suite on ubuntu with mostly ubuntu packages (ubuntu-18.04, true, false)	1s
run test suite on debian latest (32 bit)	35s
unix-like, conda (ubuntu-18.04, 3.8, false, false)	1s
run test suite on windows using Miniconda for package management (windows-2019, false, false)	4m 28s
unix-like, conda (ubuntu-18.04, 3.9, false, false)	1s
run test suite on windows using Miniconda for package management (windows-2016, true, false)	1s
unix-like, conda (ubuntu-latest, 3.7, false, false)	1m 58s
run test suite on windows using Miniconda for package management (windows-2022, false, true)	3m 18s
unix-like, conda (ubuntu-latest, 3.8, false, false)	1m 55s
unix-like, conda (ubuntu-latest, 3.9, false, false)	1m 52s
unix-like, conda (macos-10.15, 3.7, false, false)	1s
unix-like, conda (macos-10.15, 3.8, false, false)	1s
unix-like, conda (macos-10.15, 3.9, false, false)	1s
unix-like, conda (macos-latest, 3.7, false, false)	4m 5s
unix-like, conda (macos-latest, 3.8, false, false)	4m 56s
unix-like, conda (macos-latest, 3.9, false, false)	4m 42s
unix-like, conda (ubuntu-18.04, 3.6, true, false)	1s
unix-like, conda (ubuntu-latest, 3.6, true, false)	7s
unix-like, conda (macos-10.15, 3.6, true, false)	1s
unix-like, conda (macos-latest, 3.6, true, false)	3m 33s
	31m 55s

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