Use importlib_metadata for 3.8 and 3.9 only #229

Job	Run time
opentelemetry-exporter-opencensus	40s
opentelemetry-exporter-otlp-combined	50s
opentelemetry-api	42s
opentelemetry-exporter-otlp-proto-common	48s
opentelemetry-getting-started	1m 6s
opentelemetry-opencensus-shim	42s
opentelemetry-opentracing-shim	41s
opentelemetry-proto	31s
opentelemetry-semantic-conventions	33s
opentelemetry-exporter-otlp-proto-grpc	50s
opentelemetry-exporter-otlp-proto-http	51s
opentelemetry-exporter-prometheus	42s
opentelemetry-exporter-zipkin-combined	39s
opentelemetry-exporter-zipkin-json	38s
opentelemetry-exporter-zipkin-proto-http	48s
opentelemetry-propagator-b3	38s
opentelemetry-propagator-jaeger	38s
opentelemetry-test-utils	38s
opentelemetry-sdk	50s
	13m 45s

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