def schema_key_for(self, seq_no: int) -> SchemaKey:
LOGGER.debug('<STR_LIT>', seq_no)<EOL>rv = self._seq_no2schema_key.get(seq_no, None)<EOL>LOGGER.debug('<STR_LIT>', rv)<EOL>return rv<EOL>
Get schema key for schema by sequence number if known, None for no such schema in cache.

:param seq_no: sequence number
:return: corresponding schema key or None