simple_config: allow deepcopy-ing ConfigVars

Was getting error:
```
>>> import copy
>>> from electrum.simple_config import SimpleConfig
>>> copy.deepcopy(SimpleConfig.EXPERIMENTAL_LN_FORWARD_PAYMENTS)
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
  File "...\Python\Python310\lib\copy.py", line 161, in deepcopy
    rv = reductor(4)
TypeError: cannot pickle 'ConfigVar' object
```

This is useful in tests/test_lnpeer.py, to deepcopy the GRAPH_DEFINITIONS dict.

electrum/simple_config.py

@@ -89,6 +89,10 @@ class ConfigVar(property):
     def __repr__(self):
         return f"<ConfigVar key={self._key!r}>"
 
+    def __deepcopy__(self, memo):
+        cv = ConfigVar(self._key, default=self._default, type_=self._type)
+        return cv
+
 
 class ConfigVarWithConfig: