Add pyproject.toml

[rossbar]

I took inspiration here from networkx/networkx#6774. I was particularly interested in testing out setuptools.build_meta as a build backend for binary extensions. In pygraphviz's case, this seems to have been entirely straightforward (on Linux at least). I think one of the main reasons why is that the build actually doesn't directly depend on SWIG - the SWIG-generated source files are actually tracked in the source repository, so SWIG is not actually required as a build dependency.

This means that updating SWIG doesn't actually have an effect on the package unless swig is actually run to re-generate the sources. This is something we can/probably should fix, but is a separate issue.

Aside from that, there are still plenty of other little TODO's (i.e. optional dependencies/templating pyproject.toml as was done in networkx/networkx#6774, etc.) but hopefully this is a decent starting point!

[codecov-commenter]

Codecov Report

All modified and coverable lines are covered by tests ✅

Comparison is base (0b466b6) 81.02% compared to head (be5b6f1) 81.02%.

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Additional details and impacted files

@@           Coverage Diff           @@
##             main     #471   +/-   ##
=======================================
  Coverage   81.02%   81.02%           
=======================================
  Files           5        5           
  Lines        1254     1254           
=======================================
  Hits         1016     1016           
  Misses        238      238           

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[rossbar]

The next major step is to figure out how best to workaround the deprecation of --global-options for builds on systems where the library path and/or header paths need to be specified (e.g. windows). One solution appears to be defining a custom backend - see e.g. python-pillow/Pillow#7171.

This feels like a lot of extra code to have to add for this feature... ideally the setuptools.backend_meta would add built-in support for these Extension options, but from my limited reading I can't get a good sense of what the current state of affairs is... more investigation required. FWIW this "just works" on the systems where graphviz is installed in a standard location with the necessary libraries/headers already installed in locations covered by LIBDIR and INCLUDEDIR.

[rossbar]

Shoot... I just saw the other merge push and assumed it was just merge stuff so I rebased on main and force-pushed over it. Then it occurred to me that there may have been other, non-merge-conflict changes in there, in which case I just bulldozed them. Apologies if that was the case! I'm stepping AFK soon so feel free to push to this as necessary and sorry again if I overwrote changes!

[rossbar]

Okay I've rebased on main and this is now working on all platforms with the updated --config-settings flags from #484.

One of the most important things to double-check during review is that I ported over all the desired metadata from the setup.py file.

[jarrodmillman]

LGTM. I will make a release candidate and double check that we didn't break anything or leave out any metadata.

@rossbar I will let you merge.