Allow dispatch of the etuple function

The `etuple` function returns an instance of the `ExpressionTuple`
class. If we wish to return an instance of a subclass of
`ExpressionTuple` we need to implement a new function. This allows one
to dispatch the `etuple` function to keep a uniform API.

etuples/core.py

@@ -5,6 +5,8 @@
 from collections.abc import Generator, Sequence
 from typing import Callable
 
+from multipledispatch import dispatch
+
 etuple_repr = reprlib.Repr()
 etuple_repr.maxstring = 100
 etuple_repr.maxother = 100
@@ -337,6 +339,7 @@ def __hash__(self):
         return hash(self._tuple)
 
 
+@dispatch([object])
 def etuple(*args, **kwargs):
     """Create an ExpressionTuple from the argument list.