Homebrew installations of python throw an error on building

When building with a homebrew installation of python an error is raised. We can avoid this by installing in a virtual environment as suggested by the error message. For example
```
>> which python3
>> /opt/homebrew/bin/python3

>> python3 -m pip install .
>> error: externally-managed-environment

× This environment is externally managed
╰─> To install Python packages system-wide, try brew install
xyz, where xyz is the package you are trying to
install.

If you wish to install a non-brew-packaged Python package,
create a virtual environment using python3 -m venv path/to/venv.
Then use path/to/venv/bin/python and path/to/venv/bin/pip.

If you wish to install a non-brew packaged Python application,
it may be easiest to use pipx install xyz, which will manage a
virtual environment for you. Make sure you have pipx installed.

note: If you believe this is a mistake, please contact your Python installation or OS distribution provider. You can override this, at the risk of breaking your Python installation or OS, by passing --break-system-packages.
hint: See PEP 668 for the detailed specification.
```

It is easy to resolve this by doing (in an appropriate directory, eg. assume the following is in project root).
I propose to add the following notes to the build and installation part of the readme.
```
python3 -m venv venv
source venv/bin/activate
python3 -m pip install .
python3 -m pip install tox
tox
```