kunit: tool: fix typecheck errors about loading qemu configs

Currently, we have these errors:
$ mypy ./tools/testing/kunit/*.py
tools/testing/kunit/kunit_kernel.py:213: error: Item "_Loader" of "Optional[_Loader]" has no attribute "exec_module"
tools/testing/kunit/kunit_kernel.py:213: error: Item "None" of "Optional[_Loader]" has no attribute "exec_module"
tools/testing/kunit/kunit_kernel.py:214: error: Module has no attribute "QEMU_ARCH"
tools/testing/kunit/kunit_kernel.py:215: error: Module has no attribute "QEMU_ARCH"

exec_module
===========

pytype currently reports no errors, but that's because there's a comment
disabling it on 213.

This is due to https://github.com/python/typeshed/pull/2626.
The fix is to assert the loaded module implements the ABC
(abstract base class) we want which has exec_module support.

QEMU_ARCH
=========

pytype is fine with this, but mypy is not:
https://github.com/python/mypy/issues/5059

Add a check that the loaded module does indeed have QEMU_ARCH.
Note: this is not enough to appease mypy, so we also add a comment to
squash the warning.

Signed-off-by: Daniel Latypov <dlatypov@google.com>
Reviewed-by: David Gow <davidgow@google.com>
Reviewed-by: Brendan Higgins <brendanhiggins@google.com>
Signed-off-by: Shuah Khan <skhan@linuxfoundation.org>

tools/testing/kunit/kunit_kernel.py

@@ -207,12 +207,15 @@ def get_source_tree_ops_from_qemu_config(config_path: str,
 	module_path = '.' + os.path.join(os.path.basename(QEMU_CONFIGS_DIR), os.path.basename(config_path))
 	spec = importlib.util.spec_from_file_location(module_path, config_path)
 	config = importlib.util.module_from_spec(spec)
-	# TODO(brendanhiggins@google.com): I looked this up and apparently other
-	# Python projects have noted that pytype complains that "No attribute
-	# 'exec_module' on _importlib_modulespec._Loader". Disabling for now.
-	spec.loader.exec_module(config) # pytype: disable=attribute-error
-	return config.QEMU_ARCH.linux_arch, LinuxSourceTreeOperationsQemu(
-			config.QEMU_ARCH, cross_compile=cross_compile)
+	# See https://github.com/python/typeshed/pull/2626 for context.
+	assert isinstance(spec.loader, importlib.abc.Loader)
+	spec.loader.exec_module(config)
+
+	if not hasattr(config, 'QEMU_ARCH'):
+		raise ValueError('qemu_config module missing "QEMU_ARCH": ' + config_path)
+	params: qemu_config.QemuArchParams = config.QEMU_ARCH  # type: ignore
+	return params.linux_arch, LinuxSourceTreeOperationsQemu(
+			params, cross_compile=cross_compile)
 
 class LinuxSourceTree(object):
 	"""Represents a Linux kernel source tree with KUnit tests."""