Use the non-legacy setuptools backend in pyproject.toml

Use the regular `setuptools.build_meta` backend rather than
the `__legacy__` backend.  The latter was never meant to be used
explicitly in `pyproject.toml` files but rather as implicit fallback
in tools such as pip(1).  See e.g.:
pypa/setuptools#1689

Signed-off-by: Michał Górny <[email protected]>

Author: mgorny

pyproject.toml

@@ -1,6 +1,6 @@
 [build-system]
 requires = ["setuptools>=40.8.0"]
-build-backend = "setuptools.build_meta:__legacy__"
+build-backend = "setuptools.build_meta"
 
 # TODO: Remove when https://github.com/mgedmin/check-manifest/pull/155 released
 [tool.check-manifest]