対処前

 % python --version
Python 2.7.16

対処方法
pyenv initを実行

% pyenv init

# (The below instructions are intended for common
# shell setups. See the README for more guidance
# if they don't apply and/or don't work for you.)

# Add pyenv executable to PATH and
# enable shims by adding the following
# to ~/.profile and ~/.zprofile:

export PYENV_ROOT="$HOME/.pyenv"
export PATH="$PYENV_ROOT/bin:$PATH"
eval "$(pyenv init --path)"

# Load pyenv into the shell by adding
# the following to ~/.zshrc:

eval "$(pyenv init -)"

# Make sure to restart your entire logon session
# for changes to profile files to take effect.

次にzprofileを開き、

 % open ~/.zprofile

下記を記入

export PYENV_ROOT="$HOME/.pyenv"
export PATH="$PYENV_ROOT/bin:$PATH"
eval "$(pyenv init --path)"

次にzshrc を開く

% open ~/.zshrc

下記を記入

eval "$(pyenv init -)"

次に下記を実行し、保存すると

% source ~/.zprofile    
% source ~/.zshrc

改善された

% pyenv global 3.9.6
% python --version  
Python 3.9.6

ただし、

 % open ~/.zprofile   
The file /Users/yuuki/.zprofile does not exist.

の場合

% echo 'export PYENV_ROOT="$HOME/.pyenv"' >> ~/.zprofile
% echo 'export PATH="$PYENV_ROOT/bin:$PATH"' >> ~/.zprofile
% echo 'eval "$(pyenv init -)"' >> ~/.zprofile