twist conch --auth=sshkey doesn't work due to passing bytes to getpwnampwd.

[twm]

Just saw this on 22.8.0.rc1, with the equivalent of twist conch --auth=sshkey .

2022-08-29T07:31:45+0000 [twisted.conch.ssh.userauth.SSHUserAuthServer#critical] Error checking auth for user b'user'
	Traceback (most recent call last):
	  File "/code/env/lib/pypy3.8/site-packages/twisted/conch/ssh/userauth.py", line 285, in auth_publickey
	    return self.portal.login(c, None, interfaces.IConchUser)
	  File "/code/env/lib/pypy3.8/site-packages/twisted/cred/portal.py", line 120, in login
	    ).addCallback(self.realm.requestAvatar, mind, *interfaces)
	  File "/code/env/lib/pypy3.8/site-packages/twisted/internet/defer.py", line 531, in addCallback
	    return self.addCallbacks(callback, callbackArgs=args, callbackKeywords=kwargs)
	  File "/code/env/lib/pypy3.8/site-packages/twisted/internet/defer.py", line 511, in addCallbacks
	    self._runCallbacks()
	--- <exception caught here> ---
	  File "/code/env/lib/pypy3.8/site-packages/twisted/internet/defer.py", line 892, in _runCallbacks
	    current.result, *args, **kwargs
	  File "/code/env/lib/pypy3.8/site-packages/twisted/conch/ssh/userauth.py", line 208, in _ebMaybeBadAuth
	    reason.trap(error.NotEnoughAuthentication)
	  File "/code/env/lib/pypy3.8/site-packages/twisted/python/failure.py", line 480, in trap
	    self.raiseException()
	  File "/code/env/lib/pypy3.8/site-packages/twisted/python/failure.py", line 504, in raiseException
	    raise self.value.with_traceback(self.tb)
	  File "/code/env/lib/pypy3.8/site-packages/twisted/internet/defer.py", line 892, in _runCallbacks
	    current.result, *args, **kwargs
	  File "/code/env/lib/pypy3.8/site-packages/twisted/conch/unix.py", line 49, in requestAvatar
	    user = UnixConchUser(username)
	  File "/code/env/lib/pypy3.8/site-packages/twisted/conch/unix.py", line 57, in __init__
	    self.pwdData = pwd.getpwnam(self.username)
	builtins.TypeError: expected str, got bytes object

Originally posted by @glyph in #10286 (comment)

Refer to the patch for #10286 for UserDB typing and mocks that will be helpful.

[glyph]

OK, so, if I could summarize the history here:

While it has a very similar consequence to #10335 / #10286 (which were duplicates of each other), those were about UNIXAuthorizedKeysFiles passing bytes to getpwnam, whereas this is about UnixConchUser passing bytes to getpwnam.