Bind method may easily fail on Unix if there is no permission to bind to a privileged port:

>>> try: TCPServer(("", 80), ...)
... except Exception as err: err
... 
PermissionError(13, 'Permission denied')
>>> gc.collect()
__main__:1: ResourceWarning: unclosed <socket.socket fd=3, family=AddressFamily.AF_INET, type=SocketType.SOCK_STREAM, proto=0, laddr=('0.0.0.0', 0)>
0

This problem is inherited by HTTPServer and WSGIServer. My current workaround includes this code in a BaseServer fixup mixin, invoking server_close() if __init__() fails:

class Server(BaseServer, Context):
    def __init__(self, ...):
        try:
            super().__init__((host, port), RequestHandlerClass)
        except:  # Workaround for socketserver.TCPServer leaking socket
            self.close()
            raise
    
    def close(self):
        return self.server_close()

Patch attached.
The test wouldn't result in FD exhaustion on CPython because of the reference counting, but should still trigger RessourceWarning.

Patch looks of to me.

New changeset 437002018d2d by Charles-François Natali in branch '2.7':
Issue #22435: Fix a file descriptor leak when SocketServer bind fails.
https://hg.python.org/cpython/rev/437002018d2d

New changeset 9c8016af2ed8 by Charles-François Natali in branch '3.4':
Issue #22435: Fix a file descriptor leak when SocketServer bind fails.
https://hg.python.org/cpython/rev/9c8016af2ed8

New changeset 3bd0f2516445 by Charles-François Natali in branch 'default':
Issue #22435: Fix a file descriptor leak when SocketServer bind fails.
https://hg.python.org/cpython/rev/3bd0f2516445

This is fixed in 3.4.3. I think it can be closed.