Ignore 'import-not-found' for _validate_pyproject

Author: abravalheri

mypy.ini

@@ -31,5 +31,5 @@ ignore_missing_imports = True
 # - pkg_resources tests create modules that won't exists statically before the test is run.
 #   Let's ignore all "import-not-found" since, if an import really wasn't found, then the test would fail.
 # - setuptools._vendor.packaging._manylinux: Mypy issue, this vendored module is already excluded!
-[mypy-pkg_resources.tests.*,setuptools._vendor.packaging._manylinux]
+[mypy-pkg_resources.tests.*,setuptools._vendor.packaging._manylinux,setuptools.config._validate_pyproject.*]
 disable_error_code = import-not-found