Add TryFrom for AttrValue #208

Job	Run time
rustfmt	2m 38s
static builds (ubuntu-latest, stable)	2m 0s
static builds (windows-latest, stable-msvc)	8m 37s
Address sanitizer	2m 32s
conda (ubuntu, conda-forge, stable)	1m 17s
static builds (windows-latest, stable-gnu)	11m 43s
test apt (stable)	56s
conda (windows, conda-forge, stable)	3m 36s
test apt (beta)	57s
static builds (macos-11, stable)	3m 4s
test apt (nightly)	56s
conda (macos, conda-forge, stable)	2m 39s
static builds (macos-latest, stable)	2m 29s
	43m 24s

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