support path names with spaces

[bertsky]

When opening a workspace with spaces anywhere in the directory names, ocrd_browser fails:

browse-ocrd PRImA\ Layout\ Analysis\ Dataset/mets.xml
FileNotFoundError: File path passed as 'url' to download_to_directory does not exist: /daten/PRImA%20Layout%20Analysis%20Dataset/mets.xml

The cause is not on the OCR-D side AFAICS, but here:

browse-ocrd/ocrd_browser/model/document.py

Line 75 in 73fc4e2

mets_url = cls._strip_local(mets_url)

which in turn does

browse-ocrd/ocrd_browser/model/document.py

Lines 496 to 497 in 73fc4e2

	def _strip_local(mets_url: Union[Path, str], disallow_remote: bool = True) -> str:
	result = urlparse(str(mets_url))