Open a Terminal, and launch following command to install Python3 from Apple:

xcode-select --install

If system asks about install Developer Tools for macOS, do it.

Next, update pip (Python package manager) and install ESPTool:

python3 -m pip install --upgrade pip
python3 -m pip install esptool

In order to launch esptool.py, exec directly with this:

python3 -m esptool

Alternatively, but not needed, you can add Python binaries directory to PATH:

~/Library/Python/3.9/bin

To update a current installation of ESPTool to last version:

python3 -m pip install --upgrade esptool