Skip a test case when no internet connection is available

[stratakis]

When trying to build the latest version of xmlschema on Fedora test_pl_validation_translation fails as it requires an internet connection.

logs:

test_pl_validation_translation (tests.test_translations.TestTranslations.test_pl_validation_translation) ... /builddir/build/BUILD/xmlschema-2.4.0/tests/test_translations.py:122: XMLSchemaImportWarning: Import of namespace 'http://crd.gov.pl/xml/schematy/dziedzinowe/mf/2020/07/06/eD/DefinicjeTypy/' from ['http://crd.gov.pl/xml/schematy/dziedzinowe/mf/2020/07/06/eD/DefinicjeTypy/StrukturyDanych_v7-0E.xsd'] failed: cannot access to resource 'http://crd.gov.pl/xml/schematy/dziedzinowe/mf/2020/07/06/eD/DefinicjeTypy/StrukturyDanych_v7-0E.xsd': [Errno -3] Temporary failure in name resolution.
  schema = XMLSchema(
/builddir/build/BUILD/xmlschema-2.4.0/tests/test_translations.py:122: XMLSchemaImportWarning: Import of namespace 'http://crd.gov.pl/xml/schematy/struktura/2009/11/16/' from ['http://crd.gov.pl/xml/schematy/struktura/2009/11/16/struktura.xsd'] failed: cannot access to resource 'http://crd.gov.pl/xml/schematy/struktura/2009/11/16/struktura.xsd': [Errno -3] Temporary failure in name resolution.
  schema = XMLSchema(
/builddir/build/BUILD/xmlschema-2.4.0/tests/test_translations.py:122: XMLSchemaImportWarning: Import of namespace 'http://crd.gov.pl/xml/schematy/dziedzinowe/mf/2021/04/14/eTW/Commons/' from ['http://crd.gov.pl/xml/schematy/dziedzinowe/mf/2021/04/14/eTW/Commons/schemat_commons.xsd'] failed: cannot access to resource 'http://crd.gov.pl/xml/schematy/dziedzinowe/mf/2021/04/14/eTW/Commons/schemat_commons.xsd': [Errno -3] Temporary failure in name resolution.
  schema = XMLSchema(
FAIL