Fix how Haskell to Python opcode generation handles unit. It seems
that, in 'template-haskell-2.7.0', unit is represented by Template
Haskell as a constructor of the form '(ConT name)', where 'name' is
the constructor name for 'Unit'.  However, in 'template-haskell-2.8.0'
unit is represented as a tuple with no elements (i.e., 'TupleT 0').
This patch fixes the opcode generation to handle both cases, not just
the first.

Signed-off-by: Jose A. Lopes <jabolopes@google.com>
Reviewed-by: Klaus Aehlig <aehlig@google.com>