Make the dependency on sympy be a version-pinned dependency
Make the dependency on sympy be a version-pinned dependency

All the other existing dependencies are version-pinned to ensure
reproducibility, so make the recently-added dependency on sympy be
version-pinned too for the same reason and for consistency.

bmaluenda/switch,OCM-Lab-PUC/switch-chile,OCM-Lab-PUC/switch-chile