docs: def from_file(pkg_file):
          """
          Return a |PackageReader| instance loaded with contents of *pkg_file*.
          """
          phys_reader = PhysPkgReader(pkg_file)
          content_types = _ContentTypeMap.from_xml(phys_reader.content_types_xml)
          pkg_srels = PackageReader._srels_for(phys_reader, PACKAGE_URI)
          sparts = PackageReader._load_serialized_parts(
              phys_reader, pkg_srels, content_types
          )
          phys_reader.close()
          return PackageReader(content_types, pkg_srels, sparts)