These characters are not allowed and trying to create a Zulip message
with those characters throws a JsonableError in check_stream_topic.
We don't want to reject emails with those chars in the subject, so
it's best to just modify it appropriately.
Under the unicodedata distributed with Python 3.6, some Emoji are
classified as `Cn`, and not `So`:
```
$ unicode 1f929 --long
U+1F929 GRINNING FACE WITH STAR EYES
UTF-8: f0 9f a4 a9 UTF-16BE: d83edd29 Decimal: 🤩 Octal: \0374451
🤩
Category: So (Symbol, Other); East Asian width: W (wide)
Unicode block: 1F900..1F9FF; Supplemental Symbols and Pictographs
Bidi: ON (Other Neutrals)
$ python3.6 -c 'import unicodedata; print(unicodedata.category("\U0001f929"))'
Cn
$ python3.7 -c 'import unicodedata; print(unicodedata.category("\U0001f929"))'
So
```
Drop `Cn` from the list of excluded Unicode character classes, and
replace it with an explicit list of the 66 non-characters, which are
invariant.
Co-authored-by: Shlok Patel <shlokcpatel2001@gmail.com>
Mateusz Mandera
Alex Vandiver
Anders Kaseorg