brew upgrade python3 升级Python更高版

查看错误日志，看是哪个依赖库下载错误

curl: (18) transfer closed with 1071091 bytes remaining to read
Error: An exception occurred within a child process:
  DownloadError: Failed to download resource "python--pip"
Download failed: https://files.pythonhosted.org/packages/ce/ea/9b445176a65ae4ba22dce1d93e4b5fe182f953df71a145f557cffaffc1bf/pip-19.3.1.tar.gz

手动下载该依赖库，并复制到~/Library/Caches/Homebrew/downloads/里：
变更文件名前缀与对应的.incomplete缓存文件前缀一致
拷贝成功后删除与之对应的.incomplete缓存文件

$ cd ~/Library/Caches/Homebrew/downloads/
$ ls
0b4dcf92e1664a17fefc4c0f7993a9943849d88144a7a22c4b5458271c1527ea--pip-19.3.1.tar.gz.incomplete
$ cp ~/Downloads/pip-19.3.1.tar.gz ./0b4dcf92e1664a17fefc4c0f7993a9943849d88144a7a22c4b5458271c1527ea--pip-19.3.1.tar.gz
$ rm -rf 0b4dcf92e1664a17fefc4c0f7993a9943849d88144a7a22c4b5458271c1527ea--pip-19.3.1.tar.gz.incomplete