pip安装greenlet的时候，得到了以下提示：

Building wheels for collected packages: greenlet
  Building wheel for greenlet (setup.py) ... error
  ERROR: Command errored out with exit status 1:
   command: /System/Library/Frameworks/Python.framework/Versions/2.7/Resources/Python.app/Contents/MacOS/Python -u -c 'import sys, setuptools, tokenize; sys.argv[0] = '"'"'/private/var/folders/bb/ppz02_zn03bcv85_f2k58rtr0000gp/T/pip-install-CbNfpB/greenlet/setup.py'"'"'; __file__='"'"'/private/var/folders/bb/ppz02_zn03bcv85_f2k58rtr0000gp/T/pip-install-CbNfpB/greenlet/setup.py'"'"';f=getattr(tokenize, '"'"'open'"'"', open)(__file__);code=f.read().replace('"'"'\r\n'"'"', '"'"'\n'"'"');f.close();exec(compile(code, __file__, '"'"'exec'"'"'))' bdist_wheel -d /private/var/folders/bb/ppz02_zn03bcv85_f2k58rtr0000gp/T/pip-wheel-rc0XGE
       cwd: /private/var/folders/bb/ppz02_zn03bcv85_f2k58rtr0000gp/T/pip-install-CbNfpB/greenlet/
  Complete output (9 lines):
  running bdist_wheel
  running build
  running build_ext
  building 'greenlet' extension
  creating build
  creating build/temp.macosx-11.1-x86_64-2.7
  cc -fno-strict-aliasing -fno-common -dynamic -g -Os -pipe -fno-common -fno-strict-aliasing -fwrapv -DENABLE_DTRACE -DMACOSX -DNDEBUG -Wall -Wstrict-prototypes -Wshorten-64-to-32 -iwithsysroot /usr/local/libressl/include -DNDEBUG -g -fwrapv -Os -Wall -Wstrict-prototypes -DENABLE_DTRACE -pipe -I/System/Library/Frameworks/Python.framework/Versions/2.7/include/python2.7 -c greenlet.c -o build/temp.macosx-11.1-x86_64-2.7/greenlet.o
  clang: error: invalid version number in 'MACOSX_DEPLOYMENT_TARGET=11.1'
  error: command 'cc' failed with exit status 1
  ----------------------------------------
  ERROR: Failed building wheel for greenlet
  Running setup.py clean for greenlet
Failed to build greenlet

得到其中的关键提示：

clang: error: invalid version number in 'MACOSX_DEPLOYMENT_TARGET=11.1'

大意是clang的版本跟MACOX的版本不太对应，或许需要更新clang。

检查一下更新情况：

% brew doctor
Please note that these warnings are just used to help the Homebrew maintainers
with debugging if you file an issue. If everything you use Homebrew for is
working fine: please don't worry or file an issue; just ignore this. Thanks!

Warning: Suspicious https://github.com/Homebrew/brew git origin remote found.
The current git origin is:
  https://mirrors.ustc.edu.cn/brew.git

With a non-standard origin, Homebrew won't update properly.
You can solve this by setting the origin remote:
  git -C "/usr/local/Homebrew" remote set-url origin https://github.com/Homebrew/brew


Warning: A newer Command Line Tools release is available.
Update them from Software Update in System Preferences or run:
  softwareupdate --all --install --force

If that doesn't show you an update run:
  sudo rm -rf /Library/Developer/CommandLineTools
  sudo xcode-select --install

Alternatively, manually download them from:
  https://developer.apple.com/download/more/.


Warning: Suspicious https://github.com/Homebrew/homebrew-core git origin remote found.
The current git origin is:
  https://mirrors.ustc.edu.cn/homebrew-core.git

With a non-standard origin, Homebrew won't update properly.
You can solve this by setting the origin remote:
  git -C "/usr/local/Homebrew/Library/Taps/homebrew/homebrew-core" remote set-url origin https://github.com/Homebrew/homebrew-core

Warning: Your CLT does not support macOS 11.
It is either outdated or was modified.
Please update your CLT or delete it if no updates are available.
Update them from Software Update in System Preferences or run:
  softwareupdate --all --install --force

If that doesn't show you an update run:
  sudo rm -rf /Library/Developer/CommandLineTools
  sudo xcode-select --install

Alternatively, manually download them from:
  https://developer.apple.com/download/more/.


Warning: Homebrew's sbin was not found in your PATH but you have installed
formulae that put executables in /usr/local/sbin.
Consider setting the PATH for example like so:
  echo 'export PATH="/usr/local/sbin:$PATH"' >> ~/.zshrc

其中有那么一个更新提示：

Warning: A newer Command Line Tools release is available.
Update them from Software Update in System Preferences or run:
  softwareupdate --all --install --force

If that doesn't show you an update run:
  sudo rm -rf /Library/Developer/CommandLineTools
  sudo xcode-select --install

Ok，那就根据提示，更新一下好了。

% softwareupdate --all --install --force

更新后，再次执行安装，问题即解决。