[metrics-bugs] #33433 [Metrics/Onionperf]: Add error handling for older stem versions

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#33433: Add error handling for older stem versions
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 Reporter:  acute                      |          Owner:  metrics-team
     Type:  defect                     |         Status:  new
 Priority:  Medium                     |      Milestone:
Component:  Metrics/Onionperf          |        Version:
 Severity:  Normal                     |     Resolution:
 Keywords:  metrics-team-roadmap-2020  |  Actual Points:
Parent ID:  #33321                     |         Points:  0.5
 Reviewer:                             |        Sponsor:  Sponsor59-must
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Comment (by acute):

 It appears we already document the required stem version in README.md as
 1.7.0 - Attaching a patch to include this in requirements.txt

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Ticket URL: <https://trac.torproject.org/projects/tor/ticket/33433#comment:7>
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