Include `py.typed` in packaging

[mattwthompson]

Description

I thought that py.typed would be included in the packaged tarballs, but it's not. It seems like include_package_data=True misses files that are not part of modules and/or not included in MANIFEST.in. Since the conda recipe calls pip install . internally I expect this change to have the same effect there.

This conflicts somewhat with current best practices (https://github.com/pypa/sampleproject/pull/155) but removing that file from our projects is a larger lift than I want to try for today.

$ git checkout 0.0.3 && git clean -fdx && pip install . && ls $CONDA_PREFIX/lib/python3.11/site-packages/openff/models
HEAD is now at 56c30d2 Merge pull request #16 from openforcefield/openmm-optional
Removing .DS_Store
Removing __pycache__/
Removing build/
Removing openff_models.egg-info/
Processing /Users/mattthompson/software/openff-models
  Preparing metadata (setup.py) ... done
Building wheels for collected packages: openff-models
  Building wheel for openff-models (setup.py) ... done
  Created wheel for openff-models: filename=openff_models-0.0.3-py3-none-any.whl size=9630 sha256=da28c154668b00611a87cbf567120ad25253164928c17ca90cf09439c6ac3854
  Stored in directory: /Users/mattthompson/Library/Caches/pip/wheels/e0/b2/81/d7a551a5e4272049ee4de6e7956a2a05feed0ebf2752f2f91b
Successfully built openff-models
Installing collected packages: openff-models
Successfully installed openff-models-0.0.3
__init__.py   __pycache__   _version.py   exceptions.py models.py     tests         types.py
$ pip uninstall openff-models
Found existing installation: openff-models 0.0.3
Uninstalling openff-models-0.0.3:
  Would remove:
    /Users/mattthompson/mambaforge/envs/openff-models-no-openmm/lib/python3.11/site-packages/openff/models/*
    /Users/mattthompson/mambaforge/envs/openff-models-no-openmm/lib/python3.11/site-packages/openff_models-0.0.3.dist-info/*
Proceed (Y/n)? y
  Successfully uninstalled openff-models-0.0.3
$ git checkout include-py-typed && git clean -fdx && pip install . && ls $CONDA_PREFIX/lib/python3.11/site-packages/openff/models
Previous HEAD position was 56c30d2 Merge pull request #16 from openforcefield/openmm-optional
Switched to branch 'include-py-typed'
Removing __pycache__/
Removing build/
Removing openff_models.egg-info/
Processing /Users/mattthompson/software/openff-models
  Preparing metadata (setup.py) ... done
Building wheels for collected packages: openff-models
  Building wheel for openff-models (setup.py) ... done
  Created wheel for openff-models: filename=openff_models-0.0.3+1.g399b627-py3-none-any.whl size=9938 sha256=d717d4e9bcd864aec84ae4f7b739dd3156a7f0190933e4a37937850b025b3032
  Stored in directory: /Users/mattthompson/Library/Caches/pip/wheels/e0/b2/81/d7a551a5e4272049ee4de6e7956a2a05feed0ebf2752f2f91b
Successfully built openff-models
Installing collected packages: openff-models
Successfully installed openff-models-0.0.3+1.g399b627
__init__.py   __pycache__   _version.py   exceptions.py models.py     py.typed      tests         types.py