Extend supported build-backend

Describe the bug

A project using a pyproject.toml containing

build-backend = "mesonpy"

Fails with

[ERROR] --- Build system auto-detected: PEP 517 with unsupported backend mesonpy: please read template doc

Expected behavior

The build continue with pip.

Suggestion

Reading https://packaging.python.org/en/latest/glossary/#term-Build-Backend and https://packaging.python.org/en/latest/glossary/#term-Build-Frontend I understand that when a build-backend is configured but unknow, we can use pip commands.