Automatically create a virtual environment and install dependencies in it with `pulumi new` and `pulumi policy new` for Python templates.
This will save a new `virtualenv` runtime option in `Pulumi.yaml` (`PulumiPolicy.yaml` for policy packs):
```yaml
runtime:
name: python
options:
virtualenv: venv
```
`virtualenv` is the path to a virtual environment that Pulumi will use when running `python` commands.
Existing projects are unaffected and can opt-in to using this by setting `virtualenv`, otherwise, they'll continue to work as-is.
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