fix traceparent header propagation for older versions of requests (#309)
fix traceparent header propagation for older versions of requests (#309)

until fairly recently, requests used to vendor urllib3. To make
header propagation work, we need to instrument than vendored urllib3 as
well.

closes #309

beniwohli/apm-agent-python,beniwohli/apm-agent-python,beniwohli/apm-agent-python,beniwohli/apm-agent-python