python3 needs python3-venv

.ci/prep_azure.py

@@ -35,6 +35,8 @@ venv_steps = []
 
 need_to_fix_psycopg2 = False
 
+is_python3 = os.environ['PYTHONVERSION'].startswith('3')
+
 # @dw: The VSTS-shipped Pythons available via UsePythonVErsion are pure garbage,
 # broken symlinks, incorrect permissions and missing codecs. So we use the
 # deadsnakes PPA to get sane Pythons, and setup a virtualenv to install our
@@ -54,8 +56,10 @@ if ci_lib.have_apt():
         'sudo ln -fs /usr/bin/python{pv} /usr/local/bin/python{pv}'
         .format(pv=os.environ['PYTHONVERSION'])
     ])
+    if is_python3:
+        venv_steps.append('sudo apt-get -y install python3-venv')
 # TODO: somehow `Mito36CentOS6_26` has both brew and apt installed https://dev.azure.com/dw-mitogen/Mitogen/_build/results?buildId=1031&view=logs&j=7bdbcdc6-3d3e-568d-ccf8-9ddca1a9623a&t=73d379b6-4eea-540f-c97e-046a2f620483
-elif os.environ['PYTHONVERSION'].startswith('3') and ci_lib.have_brew():
+elif is_python3 and ci_lib.have_brew():
     # Mac's System Integrity Protection prevents symlinking /usr/bin
     # and Azure isn't allowing disabling it apparently: https://developercommunityapi.westus.cloudapp.azure.com/idea/558702/allow-disabling-sip-on-microsoft-hosted-macos-agen.html
     # so we'll use /usr/local/bin/python for everything