Unexpected upcasting of complex64 to complex128

[mutricyl]

Working on pandas-dev/pandas#21374 I am Coming across unexpected complex128 upcasting.

here is an example of numexpr upcasting complex64 to complex128 when python operation conserves the correct dtype.

>>> compl = 1 + 1j
>>> ne.evaluate('compl + 2').dtype
dtype('complex128')
>>> ne.evaluate('compl + compl').dtype
dtype('complex128')
>>> compl+compl
(2+2j)
>>> type(compl+compl)
<class 'complex'>
>>> type(compl+2)
<class 'complex'>

Originally posted by @mutricyl in #155 (comment)

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