platformio-core/issues/3700, and many other recent issues in that repository, are about some platformio dependency not yet supporting python 3.9. The linked issue describes a workaround for fedora 33.

My approach does not require changing settings or disabling builtins.

My setup

I use fish as main shell; in fish I use pyenv and virtualfish.

The following steps should find their equivalents in other shells too: if you know how to create a virtualenv using a specific python version, you should be able to replicate the result.

How to make PlatformIO use a specific python version

Don’t have vscode running and don’t have the platformio extension installed.
Install latest python 3.8:
```
 $ pyenv install 3.8.6
```
This might take a few minutes to compile; also might complain about missing packages. …and this is where it gets too platform specific.
Create a virtual environment for platformio/vscode:
```
 $ vf new -p 3.8 platformio
```
(I could not specify 3.8.6 but only 3.8; and virtualfish found 3.8.5 first. Later I could vf upgrade --rebuild --python 3.8 to get 3.8.6 🤷)
Activate said virtual environment and start vscode:
```
 $ vf activate platformio
 $ code
```
(The binary is called code on my system.)
Install the extension in vscode.