Allow tweaking configuration of black and isort via custom templates

[machinehead]

Is your feature request related to a problem? Please describe.
I'm using a monorepo with multiple projects, where some of the projects are generated by openapi-python-client.
I'd like to have a single configuration of black and isort at the top level of the monorepo, but having them included in pyproject.toml of generated projects breaks that.

Describe the solution you'd like
I'd like to be able to use custom templates to alter the configuration of black and isort in generated projects, while not having to copy the rest of poetry's configuration into my custom templates.

Describe alternatives you've considered
I've considered using generate once, then manually tweaking pyproject.toml and doing update afterwards. However, I don't like this approach, since it leads to having non-reproducible manual edits.

Additional context
I think this can be achieved by a very simple one-line change that also reduces some code duplication; I'll be sending a PR for this shortly.

[machinehead]

PR: #751