Avoid ast.Str on Python 3.8+ #119

Job	Run time
build (3.8, ubuntu-latest, base)	1m 5s
build (3.8, ubuntu-latest, optional)	1m 7s
build (3.8, windows-latest, base)	1m 36s
build (3.8, windows-latest, optional)	1m 26s
build (3.9, ubuntu-latest, base)	1m 1s
build (3.9, ubuntu-latest, optional)	1m 4s
build (3.9, windows-latest, base)	1m 39s
build (3.9, windows-latest, optional)	1m 35s
build (3.10, ubuntu-latest, base)	56s
build (3.10, ubuntu-latest, optional)	57s
build (3.10, windows-latest, base)	1m 37s
build (3.10, windows-latest, optional)	2m 11s
build (3.11, ubuntu-latest, base)	54s
build (3.11, ubuntu-latest, optional)	57s
build (3.11, windows-latest, base)	1m 56s
build (3.11, windows-latest, optional)	1m 39s
build (3.12, ubuntu-latest, base)	1m 6s
build (3.12, ubuntu-latest, optional)	1m 13s
build (3.12, windows-latest, base)	2m 20s
build (3.12, windows-latest, optional)	2m 9s
build (pypy-2.7, ubuntu-latest, base)	1m 7s
build (pypy-3.10, ubuntu-latest, base)	1m 27s
build (3.8, ubuntu-latest, oldest)	1m 3s
	32m 5s

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