[tor-bugs] #26811 [Core Tor/Stem]: tox fails with pip 10

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#26811: tox fails with pip 10
-------------------------------+-------------------------
     Reporter:  dmr            |      Owner:  dmr
         Type:  defect         |     Status:  assigned
     Priority:  Medium         |  Milestone:
    Component:  Core Tor/Stem  |    Version:
     Severity:  Normal         |   Keywords:  dev testing
Actual Points:                 |  Parent ID:
       Points:                 |   Reviewer:
      Sponsor:                 |
-------------------------------+-------------------------
 Trying to run `tox` in a fresh `stem` workspace (no `.tox` dir, which
 contains the virtualenvs) now fails.

 Failure output with pip 10:
 {{{
 no such option: --allow-all-external
 ERROR: InvocationError: 'path/to/pip install --allow-all-external -e .'
 }}}

 Here's a deprecation note from a tox / pip 9 virtualenv:
 {{{
 py27 runtests: commands[0] | pip install --allow-all-external -e .
 DEPRECATION: --allow-all-external has been deprecated and will be removed
 in the future. Due to changes in the repository protocol, it no longer has
 any effect.
 }}}

 With pip 10 (somewhat recently released), this `--allow-all-external` flag
 seemingly got removed.

 **atagar: I'll have a patch for this shortly! **

--
Ticket URL: <https://trac.torproject.org/projects/tor/ticket/26811>
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