hgweb: fix websub regex flag syntax on Python 3 The `websub` config section for hgweb is broken under Python 3 when using regex flags syntax (ie the optional `i` in the example from `hg help config.websub`: patternname = s/SEARCH_REGEX/REPLACE_EXPRESSION/[i] Flags are pulled out of the specified byte-string using a regular expression, and uppercased. The flags are then iterated over and passed to the `re` module using `re.__dict__[item]`, to get the object attribute of the same name from the `re` module. So on Python 2 if the `il` flags are passed, this transition looks like: `'il'` -> `'IL'` -> `'I'` -> `re.__dict__['I']` -> `re.I` However on Python 3, these are bytes objects. When we iterate over a bytes object in Python 3, instead of getting the individual characters in the string as string objects of length one, we get the integer \ value corresponding to that byte. So the same transition looks like: `b'il'` -> `b'IL'` -> `73` -> `re.__dict__[73]` -> `KeyError` This commit fixes the type mismatch by converting the bytes to a system string before iterating over each element to pass to `re`. The transition will now look like: `b'il'` -> `u'IL'` -> `u'I'` -> `re.__dict__[u'I']` -> `re.I` In addition we expand `test-websub.t` to cover the regex flag case (for both the `websub` section and `interhg`). Differential Revision: https://phab.mercurial-scm.org/D6788