Add return type Document for parse_file()

[bact]

Add a return type to spdx.parser.parse_anything.parse_file function.

Since all parse_from_file() functions from

• rdf_parser
• tagvalue_parser
• json_parser
• xml_parser and
• yaml_parser

that parse_file() is calling, are returning Document,
it should be safe to declare that parse_file() return Document as well.

This will assist type checking.