Make DocTor Python 3 compatible
DocTor depends on Stem which recently dropped Python 2 support. We should go over all the DocTor scripts and make sure they work under a Python 3 paradigm. I'll file a separate ticket for the actual migration on carinatum once we are done here.
(Metrics folks: I'll keep the same component as for #33067 (moved) which is only temporary until we finally moved to Gitlab. Sorry for the noise but I don't want to migrate to Github first to do another migration later on and I don't want to re-open Core Tor/DocTor with all that it entails. I currently don't envision more doctor tickets until we move to Gitlab than the two, so the noise is hopefully okay-ish.)
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