Fix a syntax typo

[hroncok]

This worked for now, but is SyntaxError in Python 3.9.0a6:

  File "/usr/lib/python3.9/site-packages/demjson.py", line 4853
    elif c.isalpha() or c in'_$':
                           ^
SyntaxError: invalid string prefix

(The Python change might actually be reverted before 3.9 final, but this can be fixed anyway.)

[LaikaN57]

@dmeranda bump

This seems pretty straightforward. Can we get this merged in? Is there anything else you need on the PR?