Constraint labels can break CQM (de-)serialization

[randomir]

Description

Currently CQM.to_file() encodes constraint labels with json.dumps, allowing characters that are illegal in file names, like /. That becomes a problem only when trying to extract the CQM zip file.

However, CQM.from_file() assumes forward slashes are not allowed in file names and that / is used only as (POSIX) path separator:

https://github.com/dwavesystems/dimod/blob/85f5bafd514f32585666e5de4c75db9b53444626/dimod/constrained/constrained.py#L1071-L1073

Consequently, decoding CQMs with slashes in constraint labels will fail.

Steps To Reproduce

>>> import dimod
>>> cqm = dimod.CQM()
>>> cqm.add_constraint(dimod.Binary() == 1, label='a/b')
'a/b'
>>> dimod.CQM.from_file(cqm.to_file())
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
  File "<omitted>/env311/lib/python3.11/site-packages/dimod/constrained/constrained.py", line 1078, in from_file
    label = deserialize_variable(json.loads(constraint))
                                 ^^^^^^^^^^^^^^^^^^^^^^
  File "<omitted>/lib/python3.11/json/__init__.py", line 346, in loads
    return _default_decoder.decode(s)
           ^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "<omitted>/lib/python3.11/json/decoder.py", line 337, in decode
    obj, end = self.raw_decode(s, idx=_w(s, 0).end())
               ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "<omitted>/lib/python3.11/json/decoder.py", line 353, in raw_decode
    obj, end = self.scan_once(s, idx)
               ^^^^^^^^^^^^^^^^^^^^^^
json.decoder.JSONDecodeError: Unterminated string starting at: line 1 column 1 (char 0)

Expected Behavior

Serializing CQMs with forward slashes (and some other unicode characters, like NULL) should be forbidden. Or at least such labels should be renamed, either on constraint creation, or cqm serialization.

Environment

• any OS, any python
• reproduces with latest dimod==0.12.12