htmldocck: don't let HTMLParser convert charrefs

[hdhoang]

With python 3.5, the default value for convert_charrefs became True. This broke nbsp conversion for @matches XPATH PATTERN tests.

My distro has python 3.7 as python, so I came across this failure:

12: @matches check failed
        `XPATH PATTERN` did not match
        // @matches issue_32374/struct.T.html '//*[@class="stab unstable"]' '🔬 This is a nightly-only experimental API. \(test #32374\)$'

Putting a breakpoint on this method:

    def handle_entityref(self, name):
        self.__builder.data(entitydefs[name])

shows that it was not called. I suspect this is due to the change in behaviour between python3.4 and 3.5 documented in the constructor and the method: https://docs.python.org/3/library/html.parser.html#html.parser.HTMLParser.handle_entityref

[rust-highfive]

r? @nikomatsakis

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[euclio]

I posted a more general fix for this in #57358.

[hdhoang]

thank you for updating!