Round-tripping a QuadPrecision through string is broken

[ngoldbaum]

Originally reported by @dlakaplan here: nanograv/PINT#1944 (comment)

>>> import numpy as np
>>> from numpy_quaddtype import QuadPrecision
>>> x = QuadPrecision(np.random.random())
>>> x
QuadPrecision('1.741517691496725639410669828066602e-001', backend='sleef')
>>> str(x)
'0.17415176914967256394106698280666'
>>> QuadPrecision(str(x))
QuadPrecision('1.7415176914967256394106698280666e-001', backend='sleef')
>>> x == QuadPrecision(str(x))
False

[dlakaplan]

Thanks so much for checking this.