Fix AssertionError hashing HTML blocks spread over multiple lines (#686)

[Crozzers]

This PR fixes #686.

We were using a regex to look for HTML tags, but not accounting for newlines. Used re.S flag to account for that

[Crozzers]

Root cause seems to be bf0a1e2 where the _tag_is_closed helper was changed.
Previously it would do a simple check for number of opening tags == closing tags, whereas now it checks that the closing tag is AFTER the opening one.

In both iterations the simple tag check regex is tripped up by the sample given, so the function thinks that the tag is "closed". However the extra dilligence in the new one catches that:

        close_index = text.find(f'</{tag_name}')
        open_index = text.find(f'<{tag_name}')
        # text = '<p\n'  - so open and close index == -1
        return open_index != -1 and close_index != -1 and open_index < close_index

[nicholasserra]

Thank you!