Check sympy-live.sh exists before use in postinst

Packages must not rely on files in /usr/share/doc existing and this tree can be safely pruned from the installation. This patch makes the use of sympy-live.sh in the python-sympy-doc postinst conditional on it existing.

A better solution for the long term would be to not modify files that dpkg maintains; dpkg --audit and debsums will have lots of complaints about locally modified files. Such changes are too late for bullseye, however.

(Closes: #985052)