Add TryFrom for AttrValue #207

Job	Run time
conda (ubuntu, conda-forge, stable)	1m 22s
static builds (ubuntu-latest, stable)	2m 5s
static builds (windows-latest, stable-msvc)	7m 3s
conda (windows, conda-forge, stable)	3m 26s
rustfmt	37s
test apt (stable)	54s
static builds (windows-latest, stable-gnu)	11m 34s
conda (macos, conda-forge, stable)	2m 4s
test apt (beta)	58s
Address sanitizer	2m 27s
test apt (nightly)	55s
static builds (macos-11, stable)	3m 21s
static builds (macos-latest, stable)	3m 32s
	40m 18s

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