最近，每当我尝试在 Mac OS 10.15.6 上使用 pip 安装包时，都会发生以下情况：
- 安装成功
- 现在模块永远无法加载，python 将始终返回
ModuleNotFoundError
举个例子：
✗ pip install -U setuptools wheel
Collecting setuptools
Using cached setuptools-49.6.0-py3-none-any.whl (803 kB)
Collecting wheel
Using cached wheel-0.35.1-py2.py3-none-any.whl (33 kB)
Installing collected packages: setuptools, wheel
Attempting uninstall: setuptools
Found existing installation: setuptools 47.1.0
Uninstalling setuptools-47.1.0:
Successfully uninstalled setuptools-47.1.0
Successfully installed setuptools-49.6.0 wheel-0.35.1
WARNING: You are using pip version 20.1.1; however, version 20.2.2 is available.
You should consider upgrading via the '/Users/maeln/.pyenv/versions/3.8.5/bin/python3.8 -m pip install --upgrade pip' command.
Setuptools 已成功安装。但现在，执行任何操作时都需要导入 setuptools：
pip install pycairo PyGObject
Collecting pycairo
Using cached pycairo-1.19.1.tar.gz (205 kB)
ERROR: Command errored out with exit status 1:
command: /Users/maeln/.pyenv/versions/3.8.5/bin/python3.8 -c 'import sys, setuptools, tokenize; sys.argv[0] = '"'"'/private/var/folders/8w/wqk8fyyj4k9fcq62thdz7cf00000gn/T/pip-install-cbernvg1/pycairo/setup.py'"'"'; __file__='"'"'/private/var/folders/8w/wqk8fyyj4k9fcq62thdz7cf00000gn/T/pip-install-cbernvg1/pycairo/setup.py'"'"';f=getattr(tokenize, '"'"'open'"'"', open)(__file__);code=f.read().replace('"'"'\r\n'"'"', '"'"'\n'"'"');f.close();exec(compile(code, __file__, '"'"'exec'"'"'))' egg_info --egg-base /private/var/folders/8w/wqk8fyyj4k9fcq62thdz7cf00000gn/T/pip-pip-egg-info-7zmrjjql
cwd: /private/var/folders/8w/wqk8fyyj4k9fcq62thdz7cf00000gn/T/pip-install-cbernvg1/pycairo/
Complete output (3 lines):
Traceback (most recent call last):
File "<string>", line 1, in <module>
ModuleNotFoundError: No module named 'setuptools'
----------------------------------------
ERROR: Command errored out with exit status 1: python setup.py egg_info Check the logs for full command output.
更糟的是！如果我尝试更新 pip：
✗ /Users/maeln/.pyenv/versions/3.8.5/bin/python3.8 -m pip install --upgrade pip
Collecting pip
Using cached pip-20.2.2-py2.py3-none-any.whl (1.5 MB)
Installing collected packages: pip
Attempting uninstall: pip
Found existing installation: pip 20.1.1
Uninstalling pip-20.1.1:
Successfully uninstalled pip-20.1.1
Successfully installed pip-20.2.2
✗ pip
Traceback (most recent call last):
File "/Users/maeln/.pyenv/versions/3.8.5/bin/pip", line 5, in <module>
from pip._internal.cli.main import main
ModuleNotFoundError: No module named 'pip'
我正在使用brewpython 版本（现在3.8.5）。我尝试brew doctor，brew cleanup删除已安装的所有 python 版本，仅安装 python via brew install python，最后使用pyenvnow 都无济于事。
造成这种行为的原因是什么？