The current version of Mercurial for download on MacOS is 3.8, but unfortunately, it complains about the way that jcheck.py registers the jcheck command. The warning is:

devel-warn: missing attribute 'norepo', use @command decorator to register 'jcheck'

I was able to hack the jcheck.py script to get around this by deleting the code that registered it in cmdtable at the end and inserting this code right before the @def jcheck line:

@command("jcheck",
    [("", "lax", False, "Check comments, tags and whitespace laxly"),
     ("r", "rev", [], "check the specified revision or range (default: tip)"),
     ("s", "strict", False, "check everything")],
     "hg jcheck [-r rev] [-s]")
def jcheck(ui, repo, **opts):

That seems to work fine. (I got rid of the "opts" and "help" variables used at the bottom of the file since I wasn't sure what their scope was if I defined them just before "def jcheck" so I just inlined their values instead.