mitogen.fakessh: Fix socket AttributeError on Python 2.x

> the method only exists in python3. It should be pretty easy to fix because
the call isn't necessary in python2 (descriptors are inherited by default
there).
-- https://github.com/mitogen-hq/mitogen/pull/683#issuecomment-774690069

Co-authored-by: Ryan Ofsky <ryan@ofsky.org

mitogen/fakessh.py

@@ -490,8 +490,13 @@ def run(dest, router, args, deadline=None, econtext=None):
     fakessh.name = u'fakessh.%d' % (context_id,)
 
     sock1, sock2 = socket.socketpair()
+    try:
+        # Python 3.x only
         sock1.set_inheritable(True)
         sock2.set_inheritable(True)
+    except AttributeError:
+        # Python 2.x socket objects are always inheritable
+        pass
 
     stream = mitogen.core.MitogenProtocol.build_stream(router, context_id, mitogen.context_id)
     stream.name = u'fakessh'