我正在尝试在macOS Big Sur上将Python 3.7升级到3.9。我还在尝试避免丢失在Python 3.7上安装的包，并在Python 3.9上重新安装它们
我试过用

brew install python3
brew update && brew upgrade python

其产生了

Already up-to-date.
Warning: python3 3.9.1_7 already installed

但是，当我运行python3 --version时，它会生成Python 3.7.0
这是别名的问题吗？有没有办法卸载Python 3.7并保留Python 3.9？
运行brew link python3产量

Linking /usr/local/Cellar/python@3.9/3.9.1_7... 
Error: Could not symlink bin/2to3
Target /usr/local/bin/2to3
already exists. You may want to remove it:
  rm '/usr/local/bin/2to3'

To force the link and overwrite all conflicting files:
  brew link --overwrite python@3.9

To list all files that would be deleted:
  brew link --overwrite --dry-run python@3.9