So, for anyone like me who wanted to install it with pip, but has Python 2.7 as their default (and doesn't want to change for fear of breaking things), do the following (on a Debian based system):
For Debian/jessie it worked nicely for me to just install python3-ply and python3-pip with apt-get and then just do "pip3 install xonsh". Python3 seems to coexist just fine beside the default 2.7.
On OSX I just installed it in a virtualenv with python3:
brew install python3 # will install python3 but not make it default
mkdir xonsh && cd xonsh
virtualenv -p /usr/local/bin/python3 venv
source venv/bin/activate
pip install xonsh
xonsh
curl -O https://raw.githubusercontent.com/pypa/pip/master/contrib/ge...
sudo python3.4 get-pip.py
sudo mv /usr/local/bin/pip /usr/local/bin/pip3 # optional, adjust line below if ommited
sudo pip3 install xonsh
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then:
chsh -s /usr/local/bin/xonsh <yourusername>
EDIT: Just realized I forgot a step. You have to edit /etc/shells to add /usr/local/bin/xonsh to change your shell to it using chsh.