pylupdate does not support trailing commas

Python allows trailing commas inside function calls, and in particular the Black code reformatter adds these trailing commas when breaking long calls up over multiple lines. This is a particular problem for calls to translate(), which are often lengthy. What comes out of Black is something like:

translate(
    "AddonsInstaller",
    "Unable to read data from GitHub: check your internet connection and proxy settings and try again.",
)

pylupdate does not extract this string, because upon seeing the comma after the text, it assumes there will then be a comment. When none is found, it acts as though it is a syntax error and stops the extraction for that line. The same is true if there is a comment but no pluralization.

The bug begins on line 650 of fetchtr.cpp (that is, in the version of pylupdate released with 5.15.3):

// look for comment
if ( match(Tok_Comma) && matchStringOrNone(&com)) {
    // Code to handle the case where there is a comment, etc.
} else {
    break;
}

To handle the legal-in-Python trailing comma, that conditional needs to be broken up, the break statement dropped, and the string match set to disallow None:

// look for comment 
if (match(Tok_Comma)) {
    if (matchString(&com)) { 
        // Code to handle the case where there is a comment, etc. 
    } // No else/break here, this is legal
} 

A similar fix is necessary to handle a trailing comma after a comment line, in places where there is no pluralization information, and in the final case where there is plural information, but a comma before the trailing parenthesis (though I have not verified the fix for those two cases, they don't affect the codebase I'm working in).