Fix `doctest` failures for Python 3.10+ on `AttributeError` messages #189

Job	Run time
run test suite on debian latest (32 bit)	37s
run test suite on ubuntu with mostly ubuntu packages (ubuntu-20.04, false, false)	25s
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
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)	3m 37s
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 59s
unix-like, conda (ubuntu-latest, 3.8, false, false)	2m 1s
run test suite on windows using Miniconda for package management (windows-2022, false, true)	3m 38s
unix-like, conda (ubuntu-latest, 3.9, false, false)	2m 6s
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 21s
unix-like, conda (macos-latest, 3.8, false, false)	5m 31s
unix-like, conda (macos-latest, 3.9, false, false)	5m 48s
unix-like, conda (ubuntu-18.04, 3.6, true, false)	1s
unix-like, conda (ubuntu-latest, 3.6, true, false)	3s
unix-like, conda (macos-10.15, 3.6, true, false)	1s
unix-like, conda (macos-latest, 3.6, true, false)	3m 26s
	33m 42s

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