Switch to nightly toolchain by default (at least until pyo3 0.11 is released)

Author: mtreinish

.github/workflows/test.yml

@@ -15,7 +15,6 @@ jobs:
           { 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" },
         ]
-
     steps:
       - uses: actions/checkout@v2
       - name: Set up Python ${{ matrix.python-version }}
@@ -25,7 +24,7 @@ jobs:
       - name: Install Rust
         uses: actions-rs/toolchain@v1
         with:
-          toolchain: stable
+          toolchain: nightly
           default: true
       - run: rustup set default-host ${{ matrix.platform.rust-target }}
       - name: Build without default features