add Linux testing to CI

Author: oconnor663

.github/workflows/test.yml

@@ -11,6 +11,7 @@ jobs:
       matrix:
         python-version: [3.5, 3.6, 3.7, 3.8]
         platform: [
+          { os: "ubuntu-latest",  python-architecture: "x64", rust-target: "x86_64-unknown-linux-gnu" },
           { os: "macOS-latest",   python-architecture: "x64", rust-target: "x86_64-apple-darwin" },
           { os: "windows-latest", python-architecture: "x64", rust-target: "x86_64-pc-windows-msvc" },
           { os: "windows-latest", python-architecture: "x86", rust-target: "i686-pc-windows-msvc" },