bpo-30635: Fix refleak in test_c_locale_coercion (#2126)

When checking for reference leaks, test_c_locale_coercion is run
multiple times and so _LocaleCoercionTargetsTestCase.setUpClass() is
called multiple times. setUpClass() appends new value at each call,
so it looks like a reference leak.

Moving the setup from setUpClass() to setUpModule() avoids
this, eliminating the false alarm.