Revert the removal of UTF-8 force encoding in JSON loading #248

Job	Run time
tests (3.1, 7.1)	5m 17s
tests (3.2, 7.1)	5m 12s
tests (3.2, edge)	24s
tests (head, 7.1)	17s
tests (head, edge)	24s
tests (3.3, 7.1, --enable-frozen-string-literal)	25s
	11m 59s

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