gateway: Cache rinfo for main_thread_only

This should fix a bug triggered by pytest-cov when it tries to
call _rinfo after the gateway is already busy with a remote_exec
call:

pytest-dev/pytest-xdist#1070 (comment)

src/execnet/gateway.py

@@ -31,6 +31,11 @@ def __init__(self, io: IO, spec: XSpec) -> None:
         super().__init__(io=io, id=spec.id, _startcount=1)
         self.spec = spec
         self._initreceive()
+        if self.execmodel.backend == "main_thread_only":
+            # This will cause a deadlock if there is already a running
+            # remote_exec, so cache the result before there are any
+            # other remote_exec calls.
+            self._rinfo()
 
     @property
     def remoteaddress(self) -> str: