This example uses Nix and rules_nixpkgs to provide a Python interepreter for Bazel. The Nix-provided interpreter is used both when fetching python packages (see the py_deps WORKSPACE rule) and to construct a Python toolchain (see //:BUILD.bazel).
There's a single test under app which tests some Pandas usage. Pandas depends on, among other things, libstdc++, which is not provided by default with the Nix python package. See //:python39.nix for the steps used to build an environment around a Python 3.9 interpreter. Specifically, the stdenv.cc.cc.lib nix package provides libstdc++. Removing that line from python39.nix will result in the python test failing.
Note that you'll need to install Nix separately. Follow the instructions at nixos.org.