docs: def from_interbase_coordinates(contig, start, end=None):
          '''
          Given coordinates in 0-based interbase coordinates, return a Locus
          instance.
          '''
          typechecks.require_string(contig)
          typechecks.require_integer(start)
          if end is None:
              end = start + 1
          typechecks.require_integer(end)
          contig = pyensembl.locus.normalize_chromosome(contig)
          return Locus(contig, start, end)