Exception 'signal only works in main thread' when Connection.run command with pty=True

Describe the bug When running a command with Connection.run and argument pty=True in a thread we get that error. fabric version 2.7.0

To Reproduce Steps to reproduce the behaviour (please attach a minimal example): Running the code below on fabric 2.6.0 everything work fine, while if it is run with 2.7.0 it fails due to this line https://github.com/fabric/fabric/blob/8a9336963581df8d84cc91c4191d16fe1873b4ea/fabric/runners.py#L46

from fabric import Connection
import threading
def function(connection):
    command="uptime"
    connection.run(command, warn=True, pty=True, hide=False, timeout=None)

connection_kwargs = {
    "host":  <MY_IP>,
    "user": <MY_USER>,
    "forward_agent": False,
    "connect_kwargs": {"key_filename": "<PATH TO MY KEY>"},
}
connection = Connection(**connection_kwargs)
command="uptime"

connection.run(command, warn=True, pty=True, hide=False, timeout=None)
t1=threading.Thread(target=function, args=(connection,))
t1.start()
t1.join()

Expected behaviour fabric works within a thread

Environment Bug seen in python 3.7.2 and 3.10.1

• What version of the Python interpreter are you using? Are you using an alternative interpreter such as PyPy?
• What operating system are you using both client & server-side? Ubuntu, centos and amazon linux as server and mac as client
• Are you using OpenSSH server or something else? OpenSSH
• Which version or versions of the software are you using?
  • Have you already tried the latest release? yes
  • Have you, or can you, try some older releases to pin down where the bug appeared? 2.6.0 work fine
• How can the developers recreate the bug on their end? If possible, include a copy of your code, the command you used to invoke it, and the full output of your run (if applicable.)

 13:28:46 up 3 days, 20:17,  1 user,  load average: 0.00, 0.00, 0.00
Exception in thread Thread-5 (function):
Traceback (most recent call last):
  File "/Users/giordafr/.pyenv/versions/3.10.1/lib/python3.10/threading.py", line 1009, in _bootstrap_inner
    self.run()
  File "/Users/giordafr/.pyenv/versions/3.10.1/lib/python3.10/threading.py", line 946, in run
    self._target(*self._args, **self._kwargs)
  File "/tmp/test.py", line 5, in function
    connection.run(command, warn=True, pty=True, hide=False, timeout=None)
  File "<decorator-gen-3>", line 2, in run
  File "/Users/giordafr/.pyenv/versions/parallel-cluster-tests/lib/python3.10/site-packages/fabric/connection.py", line 30, in opens
    return method(self, *args, **kwargs)
  File "/Users/giordafr/.pyenv/versions/parallel-cluster-tests/lib/python3.10/site-packages/fabric/connection.py", line 725, in run
    return self._run(self._remote_runner(), command, **kwargs)
  File "/Users/giordafr/.pyenv/versions/parallel-cluster-tests/lib/python3.10/site-packages/invoke/context.py", line 102, in _run
    return runner.run(command, **kwargs)
  File "/Users/giordafr/.pyenv/versions/parallel-cluster-tests/lib/python3.10/site-packages/fabric/runners.py", line 72, in run
    return super(Remote, self).run(command, **kwargs)
  File "/Users/giordafr/.pyenv/versions/parallel-cluster-tests/lib/python3.10/site-packages/invoke/runners.py", line 379, in run
    return self._run_body(command, **kwargs)
  File "/Users/giordafr/.pyenv/versions/parallel-cluster-tests/lib/python3.10/site-packages/invoke/runners.py", line 430, in _run_body
    self.start(command, self.opts["shell"], self.env)
  File "/Users/giordafr/.pyenv/versions/parallel-cluster-tests/lib/python3.10/site-packages/fabric/runners.py", line 46, in start
    signal.signal(signal.SIGWINCH, self.handle_window_change)
  File "/Users/giordafr/.pyenv/versions/3.10.1/lib/python3.10/signal.py", line 47, in signal
    handler = _signal.signal(_enum_to_int(signalnum), _enum_to_int(handler))
ValueError: signal only works in main thread of the main interpreter

[edsharp]

We are seeing a similar signal only works in main thread regression in behaviour in 2.7 when invoking .run with

                            "warn": True,
                            "echo": True,
                            "pty": True,

Fabric 2.6.0 worked without issue.

In both cases we were using python 3.8.

[xxtaoqi]

Fabric 2.7 has bugs, Fabric 2.6 works fine