Bump canonicaljson to version 1.4.0 (#8262)

The version 1.3.0 has a bug with unicode charecters:
```
>>> from canonicaljson import encode_pretty_printed_json
>>> encode_pretty_printed_json({'a': 'à'})
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
  File "/home/erdnaxeli/.pyenv/versions/3.6.7/lib/python3.6/site-packages/canonicaljson.py", line 96, in encode_pretty_printed_json
    return _pretty_encoder.encode(json_object).encode("ascii")
UnicodeEncodeError: 'ascii' codec can't encode character '\xe0' in position 12: ordinal not in range(128)
```

Signed-off-by: Alexandre Morignot <erdnaxeli@cervoi.se>

Co-authored-by: Alexandre Morignot <erdnaxeli@cervoi.se>

changelog.d/8262.bugfix

@@ -0,0 +1 @@
+Upgrade canonicaljson to version 1.4.0 to fix an unicode encoding issue.

synapse/python_dependencies.py

@@ -43,7 +43,7 @@ REQUIREMENTS = [
     "jsonschema>=2.5.1",
     "frozendict>=1",
     "unpaddedbase64>=1.1.0",
-    "canonicaljson>=1.3.0",
+    "canonicaljson>=1.4.0",
     # we use the type definitions added in signedjson 1.1.
     "signedjson>=1.1.0",
     "pynacl>=1.2.1",