32bit builders that run on 64bit CPUs install 64bit python packages and 32bit python cannot load them. Example: https://luci-milo.appspot.com/swarming/prod/2f698ffbe713e510/steps/python_tests/0/stdout

This is a bug in pip, explained in https://codereview.chromium.org/2043213002/
latest pip has a fix. Latest virtualenv embeds version of pip that contains the fix.

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new version of virtualenv tries to copy tcl/tk that we don't ship to our machines and it fails. Example: https://luci-milo.appspot.com/swarming/prod/2f464b3b144c7410/steps/gclient_runhooks/0/stdout

currently waiting for virtualenv PR to be merged https://github.com/pypa/virtualenv/pull/931
alternatively https://github.com/pypa/virtualenv/pull/933

if that does not happen, we can vendor and modify virtualenv in https://codereview.chromium.org/2043213002/