Rename capnp.lib.capnp.Promise to capnp.lib.capnp._Promise.

This is to prevent aliasing with capnp.includes.capnp_cpp.Promise. This
aliasing is problematic if capnp/lib/capnp.pxd uses the template definition
from capnp.includes.capnp_cpp.Promise.

Signed-off-by: Keith Rothman <537074+litghost@users.noreply.github.com>

capnp/__init__.py

@@ -51,6 +51,7 @@ from .lib.capnp import (
     _StructModule,
     _write_message_to_fd,
     _write_packed_message_to_fd,
+    _Promise as Promise,
 )
 
 add_import_hook() # enable import hook by default