Fixes fluentpython#61

Start the TCP server
```shell
% ./tcp_mojifinder.py
Building index.
Serving on ('127.0.0.1', 2323). Hit CTRL-C to stop.
```

Connect to the TCP server and send a search query which return no results and some results, e.g. `missing` and `hat`
```shell
% nc localhost 2323
?> missing
────────────────────────────────────────────────────────────────── 0 found
?> hat
U+2CD4	Ⳕ	COPTIC CAPITAL LETTER OLD COPTIC HAT
U+2CD5	ⳕ	COPTIC SMALL LETTER OLD COPTIC HAT
U+A271	ꉱ	YI SYLLABLE HAT
U+D571	핱	HANGUL SYLLABLE HAT
U+1F3A9	🎩	TOP HAT
U+1F452	👒	WOMANS HAT
U+1F920	🤠	FACE WITH COWBOY HAT
U+1F973	🥳	FACE WITH PARTY HORN AND PARTY HAT
────────────────────────────────────────────────────────────────── 8 found
?>
```

The TCP server log contains no errors and doesn't crash the TCP connection:
```
 % ./tcp_mojifinder.py
Building index.
Serving on ('127.0.0.1', 2323). Hit CTRL-C to stop.
 From ('127.0.0.1', 63756): 'missing'
   To ('127.0.0.1', 63756): 0 results.
 From ('127.0.0.1', 63756): 'hat'
   To ('127.0.0.1', 63756): 8 results.
```