clnrest: Import sys in except clause explicitly

This fixes a crash on startup of core-lightning where gevent could not
be imported. This happens before sys is imported and throws us into the
except clause which calls sys.
By importing it explicitly in the except clause we are not dependend of
the order of imports in the try bracket.

Signed-off-by: Peter Neuroth <pet.v.ne@gmail.com>

Changelog-Fixes: Plugin: clnrest crashed on startup when gevent was
missing.

plugins/clnrest/clnrest.py

@@ -26,6 +26,7 @@ try:
 except ModuleNotFoundError as err:
     # OK, something is not installed?
     import json
+    import sys
     getmanifest = json.loads(sys.stdin.readline())
     print(json.dumps({'jsonrpc': "2.0",
                       'id': getmanifest['id'],