move generation of pari bindings sources out of build_ext

[kiwifb]

I have been sitting on this one for a while as I was seeing it as a corner case that only shows up in a rare workflows. But I know think it is evidence of something done incorrectly that could affect everyone if setuptools tighten up the install process.

So, what is this supposed to fix:
In the normal production of binary wheel, you would get the sdist, build the wheel for a version of python and start from scratch for every version of python.

In Gentoo where we allow for multiple python on the system, you unfold the sdist and since building python package is essentially an out of source process, you build for all your python implementations from the same source directory (usually).

In this setup the current setup.py will generate the .pxd files for the first python implementation being built and they will be shipped for that python implementation. Further python implementations will not generate a new version of .pxd files and will use the ones generated by the first implementation - but they will not get shipped (even with package_data saying that they should be). This is essentially because the .pxd files are generated in the source tree.

This PR makes the .pxd files to be generated in the python implementation build tree instead of the source tree. This ensure that they are generated with the right version of python and always shipped.

Given the observed behaviour, I have concerns that a future version of setuptools could just stop shipping .pxd files generated in the source directory as part of the build process.

[mkoeppe]

This will make this run even on just building the sdist, which does not seem correct

[kiwifb]

I had not thought of that because I did not produce sdist off course. It may need to be in build.

[kiwifb]

I just did a basic test by adding a build class and running rebuild there. The .pxd are only generated once but are now properly shipped with each python implementation. I would prefer if .pxd where always re-generated but that may be considered an efficiency win. I will check what happens to sdist.

[kiwifb]

No, that still generate and incorporate .pxd in sdist. I may have to use MANIFEST.in instead to exclude them.

[kiwifb]

I keep forgetting. This is not about all .pxd files, it is about just auto_paridecl.pxd. The sdist is fine putting rebuild in a build class.

[kiwifb]

And auto_paridecl.pxd is also not included in sdist with my current PR.