The `setuptools` package is not part of the stdlib, but often available
in the system environment (it is a buildtime requirement).

`pkg_resources` (a package provided by `setuptools`) is used as
a runtime requirement in `opentelemetry-instrumentation`. Explicitly
listing `setuptools` as a requirement protects instrumentation from
breaking with import errors in cases where `setuptools` is not available
system-wide.

For example:

* A multi-stage Docker build where the final image does not contain
buildtime requirements.
* A build system that packages the runtime dependencies into a single
binary.

This commit pins `setuptools >= 16.0` because that is the first release
that included all 5 imports instrumentation currently relies on.