Update pyo3 to 0.8.

Author: sebpuetz

.appveyor.yml

@@ -4,7 +4,7 @@ install:
   - "SET PATH=%PYTHON%;%PYTHON%\\Scripts;%PATH%"
   - "python --version"
   - appveyor DownloadFile https://win.rustup.rs/ -FileName rustup-init.exe
-  - rustup-init -yv --default-toolchain nightly-2019-02-07 --default-host x86_64-pc-windows-msvc
+  - rustup-init -yv --default-toolchain nightly-2019-07-19 --default-host x86_64-pc-windows-msvc
   - set PATH=%PATH%;%USERPROFILE%\.cargo\bin
   - rustc -V
   - cargo -V

.travis.yml

@@ -15,14 +15,14 @@ matrix:
   fast_finish: true
   include:
     - os: linux
-      rust: nightly-2019-02-07
+      rust: nightly-2019-07-19
     - if: tag IS present
       os: osx
       osx_image: xcode9.4
-      rust: nightly-2019-02-07
+      rust: nightly-2019-07-19
     - os: osx
       osx_image: xcode10.1
-      rust: nightly-2019-02-07
+      rust: nightly-2019-07-19
 
 install:
   - |