Only release the GVL where necessary. #220

Job	Run time
ruby-versions / Generate Ruby versions	2s
build (2.5 / ubuntu-latest)	26s
build (2.5 / windows-latest)	2m 5s
build (2.6 / ubuntu-latest)	26s
build (2.6 / macos-latest)	39s
build (2.6 / windows-latest)	2m 12s
build (2.7 / ubuntu-latest)	25s
build (2.7 / macos-latest)	30s
build (2.7 / windows-latest)	1m 52s
build (3.0 / ubuntu-latest)	10s
build (3.0 / macos-latest)	16s
build (3.0 / windows-latest)	1m 15s
build (3.1 / ubuntu-latest)	15s
build (3.1 / macos-latest)	15s
build (3.1 / windows-latest)	59s
build (3.2 / ubuntu-latest)	10s
build (3.2 / macos-latest)	15s
build (3.2 / windows-latest)	1m 17s
build (3.3 / ubuntu-latest)	10s
build (3.3 / macos-latest)	24s
build (3.3 / windows-latest)	2m 55s
build (head / ubuntu-latest)	10s
build (head / macos-latest)	14s
build (truffleruby-head / ubuntu-latest)	27s
build (truffleruby-head / macos-latest)	24s
build (mswin / windows-latest)	1m 46s
build (mingw / windows-latest)	1m 3s
	21m 2s

Provide feedback