In the changes from ExternalMethod 2.13.1 -> 4.5, now ExternalMethod
define __code__ (and __defaults__) to a computed attribute calling
getFunction, but getFunction is different in our patched class, so
when unwrapMethod tries to get __code__, it will raise and mapply
will not consider the external method as being callable, so a simple
__repr__ of the ExternalMethod will be used as response body when
published.

By defining __code__ to something using our patched logic, this
problem does not happen.

Utility function taking care of the internal changes between RestrictedPython v3
and v5, keeping support for both (instead of having multiple if statements).

* Emtpy parameter.
* No argument.

See merge request nexedi/erp5!1732