Use importlib_metadata for 3.8 and 3.9 only #217

Job	Run time
opentelemetry-api	40s
opentelemetry-exporter-opencensus	45s
opentelemetry-exporter-otlp-proto-common	47s
opentelemetry-exporter-otlp-combined	47s
opentelemetry-exporter-otlp-proto-grpc	53s
opentelemetry-exporter-otlp-proto-http	54s
opentelemetry-exporter-prometheus	44s
opentelemetry-exporter-zipkin-combined	40s
opentelemetry-exporter-zipkin-json	45s
opentelemetry-exporter-zipkin-proto-http	1m 1s
opentelemetry-opencensus-shim	44s
opentelemetry-getting-started	1m 6s
opentelemetry-opentracing-shim	50s
opentelemetry-propagator-b3	38s
opentelemetry-propagator-jaeger	38s
opentelemetry-proto	33s
opentelemetry-sdk	47s
opentelemetry-semantic-conventions	36s
opentelemetry-test-utils	40s
	14m 28s

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