Tolerate __builtins__ being a dict (rather than module) in is_timeout

I'm still not sure how this happens, but somehow it does in
socket_test.test_error_is_timeout. As a result, is_timeout wouldn't get
a reference to TimeoutError, so the socket error would not be correctly
identified as a timeout.

eventlet/timeout.py

@@ -174,6 +174,11 @@ def fun(*args, **kwargs):
     return fun
 
 
+if isinstance(__builtins__, dict):  # seen when running tests on py310, but HOW??
+    _timeout_err = __builtins__.get('TimeoutError', Timeout)
+else:
+    _timeout_err = getattr(__builtins__, 'TimeoutError', Timeout)
+
+
 def is_timeout(obj):
-    py3err = getattr(__builtins__, 'TimeoutError', Timeout)
-    return bool(getattr(obj, 'is_timeout', False)) or isinstance(obj, py3err)
+    return bool(getattr(obj, 'is_timeout', False)) or isinstance(obj, _timeout_err)