Update beautifulsoup4 to 4.11.1

`charset-normalizer` is now used by default if installed instead of
`chardet` (https://pyup.io/changelogs/beautifulsoup4/#4.11.0). We do
have `charset-normalizer` installed because it's a subdependency of the
requests library, so it is being used.

This caused the `test_content_too_long_returns_400` to fail since it
now thought that the encoding of `ŵ` is `{'encoding': 'Big5',
'language': 'Chinese', 'confidence': 1.0}`.

There are two options for fixing this
- change the test content so that it doesn't just contain a single
  letter - the docs state that you shouldn't run character detection on
  very tiny content
- add `chardet` as a requirement, so that the code functions exactly the
  same as before

I've chose the first option, since this avoids adding a dependency and
we should never have messages consisting of a single character.

tests/app/v2/broadcast/sample_cap_xml_documents.py

@@ -248,5 +248,5 @@ WINDEMERE = """
 """
 
 LONG_GSM7 = WITH_PLACEHOLDER_FOR_CONTENT.format('a' * 1396)
-LONG_UCS2 = WITH_PLACEHOLDER_FOR_CONTENT.format('ŵ' * 616)
+LONG_UCS2 = WITH_PLACEHOLDER_FOR_CONTENT.format('ŵyl' * 205 + 'a')
 MISSING_AREA_NAMES = re.sub("<areaDesc>.*</areaDesc>", "<areaDesc> </areaDesc>", WAINFLEET)