[Numpy-discussion] Adding a hex version like PY_VERSION_HEX

[Matti Picus]

In PR 12074 https://github.com/numpy/numpy/pull/12074 I propose adding a 
function `version.get_numpy_version_as_hex()` which returns a hex value 
to represent the current NumPy version MAJOR.MINOR.MICRO where

v = hex(MAJOR << 24 | MINOR << 16 | MICRO)

so the current 1.15.0 would become '0x10f0000'. I also made this 
avaiable via C through `get_hex_version`. The hex version is based on 
the |PY_VERSION_HEX| macro from CPython.

Currently we have a ABI version and an API version for the numpy C-API. 
We only increment those for updated or breaking changes in the NumPy 
C-API, but not for

- changes in behavior, especially in python code

- changes in sizes of outward-facing structures like PyArray_Desc

Occasionally it is desirable to determine backward compatibility from 
the runtime version, rather than from the ABI or API versions, and 
having it as a single value makes the comparison in C easy. For instance 
this may be convenient when there is suspicion that older header files 
may have been used to create or manipulate an object directly in C (or 
via a cython optimization), and we want to verify the version used to 
create the object, or when we may want to verify de-serialized objects. 
The `numpy.lib._version.NumpyVersion` class enables version comparison 
in python, but I would prefer a single value that can be stored in a C 
struct as an integer type.

Since this is an enhancement proposal, I am bringing the idea to the 
mailing list for reactions.

Matti