_dtype_from_pep3118 is overly strict on prefixes

[bmerry]

In numpy 1.12.1, _dtype_from_pep3118 requires that modifiers are specified in the order shape then byte order then count, with at most one of each. The PEP is a bit vague, but it seems like modifiers ought to be interpreted recursively i.e. a modifier can be followed by any valid PEP 3118 specifier.

In practice this can lead to certain types failing to round-trip through a memoryview e.g.:

>>> dt = np.dtype({'formats': [np.dtype((np.dtype((np.int32, (3,))), (2,)))], 'names': ['foo']})
>>> a = np.empty(0, dt)
>>> m = memoryview(a)
>>> np.array(m)
NotImplementedError: memoryview: unsupported format T{(2)(3)i:foo:}

It also causes problems if one puts an endianness specifier at the start of the string (which should always be permitted, since the original struct module allows it). I haven't managed to get numpy to generate such a format string when wrapping an array into a memoryview, but it's causing me some problems while developing new features for pybind11:

>>> np.core._internal._dtype_from_pep3118('<(3)i')
ValueError: Unknown PEP 3118 data type specifier '(3)i'

[eric-wieser]

Seems odd to me that we implement _buffer_format_string in C, but _dtype_from_pep3118, the inverse, in python