Funny you brought this up, I just had the exact same problem. I solved it eventually, but I'm still unsure as to what caused it.
Try checking your bin/pip and bin/activate scripts. In bin/pip, look at the shebang. Is it correct? If not, correct it. Then on line ~42 in your bin/activate, check to see if your virtualenv path is right. It'll look something like this
VIRTUAL_ENV="/Users/me/path/to/virtual/environment"
If it's wrong, correct it, deactivate, then . bin/activate, and if our mutual problem had the same cause, it should work. If it still doesn't, you're on the right track, anyway. I went through the same problem solving routine as you did, which piping over and over, following the stack trace, etc.
Make absolutely sure that
/Users/kristof/VirtualEnvs/testpy3/bin/pip3
is what you want, and not referring to another similarly-named test project (I had that problem, and have no idea how it started. My suspicion is running multiple virtualenvs at the same time).
If none of this works, a temporary solution may be to, as Joe Holloway said,
Just run the virtualenv's pip with its full path (i.e. don't rely on searching the executable path) and you don't even need to activate the environment. It will do the right thing.
Perhaps not ideal, but it ought to work in a pinch.
Link to my original question:
~ Answered on 2014-01-06 16:40:50