python interface: also print argument types for regular methods
python interface: also print argument types for regular methods

Functions such as isl_val_nan take an isl_ctx as first argument and
ctypes needs to be told about this type.

In order to be able to print argument types for all methods,
extend print_argtypes to take into account callbacks.

Signed-off-by: Sven Verdoolaege <e5350bbed4977f5eb8ae1dc6abd9ae59d21ace75@kotnet.org>

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