pydot_ng tests fail

[cdyson37]

Issue description

pydot_ng cannot be built as the tests fail.

Broken on hydra too: https://hydra.nixos.org/job/nixos/release-19.03/nixpkgs.python37Packages.pydot_ng.x86_64-linux/all

Steps to reproduce

$ nix-build -E 'with import <nixpkgs> {}; python37.withPackages (p: [p.pydot_ng])'
...
running install tests
mkdir: cannot create directory ‘test/my_tests’: File exists
...

Technical details

 - system: `"x86_64-linux"`
 - host os: `Linux 4.15.0-54-generic, Ubuntu, 18.04.2 LTS (Bionic Beaver)`
 - multi-user?: `no`
 - sandbox: `yes`
 - version: `nix-env (Nix) 2.2.2`
 - channels(charlie): `"nixpkgs-19.09pre182571.e829aeefa35"`
 - nixpkgs: `/home/charlie/.nix-defexpr/channels/nixpkgs`

[stale]

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