我尝试用$ brew upgrade python从“Python 3.7.3”升级到“Python 3.8”，但是当我尝试时，brew返回：

Updating Homebrew...
==> Auto-updated Homebrew!
Updated 1 tap (homebrew/core).
==> Updated Formulae
Updated 2 formulae.

Error: python not installed

我知道python已经安装了，因为我已经用了几个月了，我可以通过运行which python3来确认这一点

/usr/bin/python3

和python3 --version，

Python 3.7.3

我不知道是什么原因造成的？
问题可能是python --version仍然指向python2，Python 2.7.16。

更新

我还确认我已经运行了brew cleanup和brew doctor。
brew info python返回

python@3.8: stable 3.8.5 (bottled)
Interpreted, interactive, object-oriented programming language
https://www.python.org/
Not installed
From: https://github.com/Homebrew/homebrew-core/blob/HEAD/Formula/python@3.8.rb
License: Python-2.0
==> Dependencies
Build: pkg-config ✔
Required: gdbm ✔, openssl@1.1 ✔, readline ✔, sqlite ✔, xz ✔
==> Caveats
Python has been installed as
  /usr/local/bin/python3

Unversioned symlinks `python`, `python-config`, `pip` etc. pointing to
`python3`, `python3-config`, `pip3` etc., respectively, have been installed into
  /usr/local/opt/python@3.8/libexec/bin

You can install Python packages with
  pip3 install <package>
They will install into the site-package directory
  /usr/local/lib/python3.8/site-packages

See: https://docs.brew.sh/Homebrew-and-Python
==> Analytics
install: 587,815 (30 days), 1,615,984 (90 days), 2,710,078 (365 days)
install-on-request: 187,760 (30 days), 307,168 (90 days), 343,355 (365 days)
build-error: 0 (30 days)