python3-license-expression: Fix the ptest failure

Fix the below ptest failure:
 self = <test_skeleton_codestyle.BaseTests testMethod=test_skeleton_codestyle>
 def test_skeleton_codestyle(self):
 """
 This test shouldn't run in proliferated repositories.
 """
 setup_cfg = configparser.ConfigParser()
 setup_cfg.read("setup.cfg")
 > if setup_cfg["metadata"]["name"] != "skeleton":
 tests/test_skeleton_codestyle.py:22:
 self = <configparser.ConfigParser object at 0x7fd87cf55110>, key = 'metadata'
 def _getitem_(self, key):
 if key != self.default_section and not self.has_section(key):
 > raise KeyError(key)
 E KeyError: 'metadata'

After the patch:
 # ./run-ptest
 [snip]
 PASS: tests/test_license_expression.py::LicensingValidateTest::test_validation_bad_syntax
 PASS: tests/test_license_expression.py::LicensingValidateTest::test_validation_exception_as_regular_key
 PASS: tests/test_license_expression.py::LicensingValidateTest::test_validation_exception_with_choice
 PASS: tests/test_license_expression.py::LicensingValidateTest::test_validation_invalid_license_exception
 PASS: tests/test_license_expression.py::LicensingValidateTest::test_validation_invalid_license_exception_strict_false
 PASS: tests/test_license_expression.py::LicensingValidateTest::test_validation_invalid_license_key
 PASS: tests/test_license_expression.py::UtilTest::test_build_licensing
 PASS: tests/test_license_expression.py::UtilTest::test_build_spdx_licensing
 PASS: tests/test_license_expression.py::UtilTest::test_get_license_key_info
 PASS: tests/test_license_expression.py::UtilTest::test_get_license_key_info_vendored
 PASS: tests/test_license_expression.py::CombineExpressionTest::test_combine_expressions_with_duplicated_elements
 PASS: tests/test_license_expression.py::CombineExpressionTest::test_combine_expressions_with_empty_input
 PASS: tests/test_license_expression.py::CombineExpressionTest::test_combine_expressions_with_or_relationship
 PASS: tests/test_license_expression.py::CombineExpressionTest::test_combine_expressions_with_regular
 PASS: tests/test_skeleton_codestyle.py::BaseTests::test_skeleton_codestyle
============================================================================
Testsuite summary
 # TOTAL: 175
 # PASS: 175
 # SKIP: 0
 # XFAIL: 0
 # FAIL: 0
 # XPASS: 0

Signed-off-by: Mingli Yu <mingli.yu@windriver.com>

Changed in v4:
Add python3-license-expression to PTESTS_FAST

Signed-off-by: Alexandre Belloni <alexandre.belloni@bootlin.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>
(cherry picked from commit 6b06ad7a)
Signed-off-by: Steve Sakoman <steve@sakoman.com>