出现这个问题Some externally hosted files were ignored (use --allow-external PIL to allow)的主要原因是PIL的一些依赖库还没有安装

需要先安装PIL的依赖库

安装步骤

（1）首先通过homebrew安装libtiff，libjpeg webp little-cms2的库

brew install libtiff libjpeg webp little-cms2

（2）然后通过pip安装PIL

pip install Pillow

Mac osx 安装PIL出现Some externally hosted files were ignored (use --allow-external PIL to allow).