add C++ headers-only distribution configuration

[ianna]

as discussed with @jpivarski this restores a config.py, but it defines only header part as follows (no libraries):

% python -m awkward._v2.config
% python -m awkward._v2.config --cflags
-std=c++17 -I/Users/yana/Projects/PR1522/awkward/awkward/src/_v2/cpp-headers
% python -m awkward._v2.config --cflags-only-I
-I/Users/yana/Projects/PR1522/awkward/awkward/src/_v2/cpp-headers
% python -m awkward._v2.config --incdir       
/Users/yana/Projects/PR1522/awkward/awkward/src/_v2/cpp-headers

[codecov]

Codecov Report

Merging #1523 (946b555) into main (0c6ae03) will decrease coverage by 2.74%.
The diff coverage is 76.88%.

Impacted Files	Coverage Δ
...awkward/_v2/_connect/rdataframe/from_rdataframe.py	0.00% <0.00%> (ø)
...c/awkward/_v2/_connect/rdataframe/to_rdataframe.py	0.00% <0.00%> (ø)
src/awkward/_v2/config.py	0.00% <0.00%> (ø)
src/awkward/_v2/types/_awkward_datashape_parser.py	47.66% <ø> (ø)
src/awkward/_v2/_connect/cling.py	25.99% <9.09%> (-0.35%)	⬇️
src/awkward/_v2/types/recordtype.py	87.06% <91.66%> (+0.83%)	⬆️
src/awkward/_v2/types/type.py	96.46% <96.10%> (-1.27%)	⬇️
src/awkward/_v2/_lookup.py	97.51% <100.00%> (+0.01%)	⬆️
src/awkward/_v2/types/__init__.py	100.00% <100.00%> (ø)
src/awkward/_v2/types/regulartype.py	90.47% <0.00%> (+4.76%)	⬆️

[jpivarski]

Only one thing: I think this file should have path src/awkward/_v2/config.py so that it can't be confused with the old one. When /_v2/ → /, it will occupy the old space, but then it will be a new major version of Awkward and we expect things like that to change.

Other than that, it's great! Go ahead and merge it when it's moved into the _v2 directory.

[ianna]

Only one thing: I think this file should have path src/awkward/_v2/config.py so that it can't be confused with the old one. When /_v2/ → /, it will occupy the old space, but then it will be a new major version of Awkward and we expect things like that to change.

Other than that, it's great! Go ahead and merge it when it's moved into the _v2 directory.

I did move the file. Now the CI does not trigger the tests...