ExtRepOfObj for IsLetterAssocWordRep: return mutable list #5709

Job	Run time
Cross compiler for s390x	28m 45s
Test in OpenBSD	9m 16s
testinstall - - ubuntu-latest	4m 27s
teststandard - - ubuntu-latest	19m 1s
testmanuals - - ubuntu-latest	5m 49s
testexpect testmockpkg testspecial test-compile testlibgap testkernel testworkspace - - ubuntu-latest	7m 14s
testpackages testinstall-loadall - - ubuntu-latest	25m 46s
makemanuals testmakeinstall - CONFIGFLAGS="--prefix=/tmp/gapprefix" NO_COVERAGE=1 - ubuntu-latest	5m 25s
teststandard - ABI=32 CONFIGFLAGS="" - ubuntu-20.04	17m 55s
testinstall - HPCGAP=yes ABI=64 - ubuntu-latest	7m 32s
testmockpkg testinstall - BOOTSTRAP_MINIMAL=yes - macos-latest	3m 13s
testbugfix - CONFIGFLAGS="--enable-memory-checking" - ubuntu-latest	28m 43s
testbuildsys testmockpkg testinstall - NO_COVERAGE=1 ABI=64 BUILDDIR=out-of-tree CONFIGFLAGS="--enable-valgrind" - ubuntu-latest	4m 18s
testbuildsys testmockpkg testinstall - NO_COVERAGE=1 ABI=32 BUILDDIR=out-of-tree CONFIGFLAGS="" - ubuntu-20.04	4m 3s
testinstall - JULIA=yes CONFIGFLAGS="--enable-debug --disable-Werror" - ubuntu-20.04	4m 55s
testmockpkg testinstall - - windows-2019	16m 7s
Send Slack notification on status change	0s
	3h 12m 29s

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