Prevent python from automatically downloading dependencies

[Ryan Schmidt]

We often have this problem that we forget to specify dependencies in a python port. Python (setuptools?) helpfully automatically downloads a temporary copy of the dependency to use, so we may not notice this until committing the port, at which point if we're looking at the buildbot logs we might notice that it fails to build on older OS versions because of SSL problems.

Here's an example:

https://build.macports.org/builders/ports-10.7_x86_64_legacy-builder/builds/37372

Is there a way we can tell python in the python portgroup that it should not attempt to automatically download dependencies, so that we would discover these problems earlier?