.arcconfig

py3: fix crash when server address is 0.0.0.0 (issue6362)

`socket.getfqdn()` assumes that the name is passed as `str` on Python 3 and
always returns `str` in this case. Mercurial passed `bytes` (but still expected
a `str` result), which worked by chance in many cases, except for e.g.
b'0.0.0.0', which was returned unchanged, breaking later code.

Instead of calling `socket.getfqdn()`, we can also use `self.server_name` from
the base `HTTPServer` class, which already stores the FQDN of the locally-bound
socket name (see `BaseHTTPServer.py` in the Python 2 stdlib and
`http/server.py` in the Python 3 stdlib).

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