Fix the py.typed marker file usage #310

Job	Run time
opentelemetry-proto	28s
opentelemetry-api	42s
opentelemetry-sdk	52s
opentelemetry-exporter-opencensus	39s
opentelemetry-exporter-otlp-combined	47s
opentelemetry-exporter-otlp-proto-common	43s
opentelemetry-exporter-otlp-proto-grpc	53s
opentelemetry-exporter-otlp-proto-http	46s
opentelemetry-exporter-prometheus	41s
opentelemetry-exporter-zipkin-combined	39s
opentelemetry-exporter-zipkin-json	37s
opentelemetry-exporter-zipkin-proto-http	44s
opentelemetry-getting-started	1m 1s
opentelemetry-opencensus-shim	40s
opentelemetry-opentracing-shim	47s
opentelemetry-propagator-b3	34s
opentelemetry-propagator-jaeger	36s
opentelemetry-semantic-conventions	39s
opentelemetry-test-utils	35s
	13m 23s

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