Only release the GVL where necessary. #235

Job	Run time
ruby-versions / Generate Ruby versions	9s
build (2.5 / ubuntu-latest)	22s
build (2.5 / windows-latest)	2m 37s
build (2.6 / ubuntu-latest)	26s
build (2.6 / macos-latest)	36s
build (2.6 / windows-latest)	2m 18s
build (2.7 / ubuntu-latest)	24s
build (2.7 / macos-latest)	30s
build (2.7 / windows-latest)	2m 10s
build (3.0 / ubuntu-latest)	13s
build (3.0 / macos-latest)	18s
build (3.0 / windows-latest)	1m 26s
build (3.1 / ubuntu-latest)	17s
build (3.1 / macos-latest)	18s
build (3.1 / windows-latest)	2m 4s
build (3.2 / ubuntu-latest)	12s
build (3.2 / macos-latest)	20s
build (3.2 / windows-latest)	1m 21s
build (3.3 / ubuntu-latest)	10s
build (3.3 / macos-latest)	15s
build (3.3 / windows-latest)	2m 7s
build (head / ubuntu-latest)	11s
build (head / macos-latest)	19s
build (truffleruby-head / ubuntu-latest)	30s
build (truffleruby-head / macos-latest)	34s
build (mswin / windows-latest)	1m 31s
build (mingw / windows-latest)	1m 3s
	22m 41s

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