XSD not bundled with ocrd_validators

[kba]

e.g.

tests/validator/test_workspace_validator.py:182: AssertionError
----------------------------- Captured stdout call -----------------------------
['Validation aborted with exception: Traceback (most recent call last):\n  File "/usr/local/lib/python3.6/site-packages/ocrd_validators/workspace_validator.py", line 141, in _validate\n    self._validate_page_xsd()\n  File "/usr/local/lib/python3.6/site-packages/ocrd_validators/workspace_validator.py", line 293, in _validate_page_xsd\n    for err in XsdPageValidator.validate(Path(ocrd_file.local_filename)).errors:\n  File "/usr/local/lib/python3.6/site-packages/ocrd_validators/xsd_page_validator.py", line 17, in validate\n    return cls.instance(XSD_PAGE_URL)._validate(doc) # pylint: disable=protected-access\n  File "/usr/local/lib/python3.6/site-packages/ocrd_validators/xsd_validator.py", line 24, in instance\n    cls._instances[schema_url] = cls(schema_url)\n  File "/usr/local/lib/python3.6/site-packages/ocrd_validators/xsd_validator.py", line 47, in __init__\n    with open(XSD_PATHS[schema_url], \'r\') as f:\nFileNotFoundError: [Errno 2] No such file or directory: \'/usr/local/lib/python3.6/site-packages/ocrd_validators/xsd/page.xsd\'\n']

https://circleci.com/gh/OCR-D/core/1290