refactor: move .python-version file to the root, increase patch version

[4141done]

Most version managers interpret the .python-version file from the root and apply the
python version specified to all child folders. Moving the .pythong-version file
to the root of the project ensures that the python version is the same for all
actions, but most importantly avoids a situation when running ./bin/setup_venv.sh
from the root where the system python will be hooked for the venv.

Proposed changes

• Move .python-version file to root
• Update python version to 3.9.12 as 3.9.6 fails to build on M1 macs. Happy to walk back this change if we think it's not a good enough reason to mess with the version required.

Example

$./bin/setup_venv.sh
# ...
$ source pulumi/python/venv/bin/activate
(venv) 
$ python --version
Python 3.9.12
(venv) 

Checklist

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• I have written my commit messages in the Conventional Commits format.
• I have read the CONTRIBUTING doc
• I have added tests (when possible) that prove my fix is effective or that my feature works
• I have checked that all unit tests pass after adding my changes
• I have updated necessary documentation
• I have rebased my branch onto master
• I will ensure my PR is targeting the master branch and pulling from my branch from my own fork