At the moment, you can't. The Anaconda people have made it seriously difficult to create large packages which are not pure Python (they simply haven't thought through how people want Anaconda to talk to system supplied tools). Your best bet is simply to install sympy for the system-supplied Python, e.g. by doing
sudo apt install python3-sympy
(which may not get you the latest version, unfortunately) or by getting the latest version through a pip install,
sudo pip3 install sympy