Add script to generate abstract.pyi stub

[AchmadFathoni]

Usage: $ python tools/generate_abstract_stub.py > talib/abstract.pyi closes #745

[mrjbq7]

Should we instead have types on the inputs

@overload
def HT_DCPHASE(real: np.ndarray) -> np.ndarray: ...
@overload
def HT_DCPHASE(real: pd.Series) -> pd.Series: ...

[AchmadFathoni]

Good idea, I make it union since after running a test for single and multi return indicator I got

function	input	output
EMA	numpy	numpy
	series	numpy
	df	series
BBANDS	numpy	list of numpy
	series	list of numpy
	df	df