CMake: PyPy Support

[ax3l]

Why is this feature important?

Among CPython, PyPy is an increasingly popular, fast Python implementation and supported by pybind11 which we use for binding code.

What is the potential impact of this feature in the community?

Broader usage of ADIOS2 in more python projects on the long term. Support of all build targets on conda-forge

Is your feature request related to a problem? Please describe.

Yes, ADIOS2 currently bails out in the CMake config step of the PyPy migration (addition) on conda-forge: conda-forge/adios2-feedstock#10

Could NOT find PythonFull (missing: PYTHON_LIBRARIES Libs)

Describe the solution you'd like and potential required effort

Potentially this affects only the CMake scripts.

Additional context

Please see the CI output in conda-forge/adios2-feedstock#10

[ax3l]

PyPy+pybind11 note: while PyPy2.7 is supported (but who needs Python2.7 these days; we at least don't), PyPy3.X seams to be not yet clear: pybind/pybind11#1720

[ax3l]

Updating to the (upcoming) pybind11 2.6.0+ release should allow builds with pypy: pybind/pybind11#2146

@chuckatkins small patch for the meantime: https://github.com/pybind/pybind11/pull/2146.patch

[ax3l]

ping @chuckatkins

[ax3l]

Hi @pnorbert, can your responsible Python binding maintainer please bump the pybind11 release to the latest version for your next release to fix this? :)

[ax3l]

Ok, looks like the problem is solved in ADIOS 2.8 🎉

[ax3l]

Ok, builds now but still ha a few issues with PyPy such as:

...
2022-09-29T22:35:21.8038903Z [ 64%] Linking CXX shared module ../../lib/pypy3.8/site-packages/adios2/adios2.pypy38-pp73.so
...
  File "/home/conda/feedstock_root/build_artifacts/adios2-split_1664490385397/work/build/lib/pypy3.8/site-packages/adios2/__init__.py", line 1, in <module>
    from .adios2 import *
ModuleNotFoundError: No module named 'adios2.adios2'

The following tests FAILED:
	653 - Bindings.Python.HighLevelAPI.Serial (Failed)
	654 - Bindings.Python.BPWriteReadTypes.Serial (Failed)
	655 - Bindings.Python.BPSelectSteps.Serial (Failed)

conda-forge/adios2-feedstock#67

I think the artifact name might be off for PyPy conventions or it simply does not get installed.