Use and recommend new pip and poetry versions with support for editable installs

During development, Meltano plugin's pip_url lets users pass arguments for an editable installation (-e path/to/package). This is useful for iterating on the tap/target while testing with Meltano.

At the moment, plugins built with the SDK have to hack their way around to accomplish the same thing because pip was not able to install packages that use pyproject.toml instead of setup.py.

However, recently released pip 21.3 has support for PEP 660. The only blocker is that the PEP 517 build backend needs to implement the build_editable hook.

The poetry-core build backend is PEP 517 compliant and has recently pushed the hook to master (see PR: https://github.com/python-poetry/poetry-core/pull/182). Once this feature is released and indirectly included in the main poetry package, we'll be able to guide users into upgrading their pip and poetry version so they can install their SDK-based plugins in editable mode without hacks.

cc @aaronsteers