Fix `doctest` failures for Python 3.10+ on `AttributeError` messages #190

Job	Run time
run test suite on ubuntu with mostly ubuntu packages (ubuntu-20.04, false, false)	16s
run test suite on ubuntu with mostly ubuntu packages (ubuntu-18.04, true, false)	1s
run test suite on debian latest (32 bit)	26s
run test suite on windows using Miniconda for package management (windows-2019, false, false)	14s
unix-like, conda (ubuntu-18.04, 3.7, false, false)	1s
run test suite on windows using Miniconda for package management (windows-2016, true, false)	1s
unix-like, conda (ubuntu-18.04, 3.8, false, false)	1s
run test suite on windows using Miniconda for package management (windows-2022, false, true)	13s
unix-like, conda (ubuntu-18.04, 3.9, false, false)	1s
unix-like, conda (ubuntu-latest, 3.7, false, false)	15s
unix-like, conda (ubuntu-latest, 3.8, false, false)	14s
unix-like, conda (ubuntu-latest, 3.9, false, false)	15s
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)	1s
unix-like, conda (macos-latest, 3.8, false, false)	1s
unix-like, conda (macos-latest, 3.9, false, false)	1s
unix-like, conda (ubuntu-18.04, 3.6, true, false)	1s
unix-like, conda (ubuntu-latest, 3.6, true, false)	8s
unix-like, conda (macos-10.15, 3.6, true, false)	1s
unix-like, conda (macos-latest, 3.6, true, false)	1s
	2m 15s

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