How do I distribute fonts with my python package using python -m build?

Problem statement

My package relies on matplotlib to use a specific font which may not be installed on the target device.
I’m trying to install ttf font files from the source distribution to the matplotlib fonts/ttf directory, after building the package.

With setuptools slowly removing parts of its CLI (python setup.py sdist/bdist_wheel) and the possibility to use build (python -m build), I decided to build my package with the latter.

What I tried

I tried modifying the setup.py from the outdated https://stackoverflow.com/a/34304823/8797886.
My question differs from this post in that I would like to use python -m build.

setup.py

import warnings
from functools import partial

from setuptools import setup
from setuptools.command.install import install

def _post_install():
    # Try to install custom fonts
    try:
        import os
        import shutil

        import matplotlib as mpl

        import mypkg

        # Find where matplotlib stores its True Type fonts
        mpl_data_dir = os.path.dirname(mpl.matplotlib_fname())
        mpl_ttf_dir = os.path.join(mpl_data_dir, "fonts", "ttf")

        # Copy the font files to matplotlib's True Type font directory
        # (I originally tried to move the font files instead of copy them,
        # but it did not seem to work, so I gave up.)
        cp_ttf_dir = os.path.join(os.path.dirname(mypkg.__file__), "ttf")
        for file_name in os.listdir(cp_ttf_dir):
            if file_name[-4:] == ".ttf":
                old_path = os.path.join(cp_ttf_dir, file_name)
                new_path = os.path.join(mpl_ttf_dir, file_name)
                shutil.move(old_path, new_path)
                print("moving" + old_path + " -> " + new_path)

        # Try to delete matplotlib's fontList cache
        mpl_cache_dir = mpl.get_cachedir()
        mpl_cache_dir_ls = os.listdir(mpl_cache_dir)
        if "fontList.cache" in mpl_cache_dir_ls:
            fontList_path = os.path.join(mpl_cache_dir, "fontList.cache")
            os.remove(fontList_path)
            print("Deleted the matplotlib fontList.cache")
    except:
        warnings.warn(
            "An issue occured while installing the custom fonts for mypkg."
        )
        raise

# Set up the machinery to install custom fonts.  Subclass the setup tools install
# class in order to run custom commands during installation.
class MoveTTF(install):
    def run(self):
        """
        Performs the usual install process and then copies the True Type fonts
        that come with shithappens into matplotlib's True Type font directory,
        and deletes the matplotlib fontList.cache.
        """
        # Perform the usual install process
        install.run(self)
        self.execute(_post_install, (), "Moving Custom Font to matplotlib.")


setup_movettf = partial(
    setup,
    package_data={"": ["ttf/*.ttf"]},
    cmdclass={"install": MoveTTF},
)

setup_movettf()

I used a partial setup, because I expect build to fill in all the other stuff from pyproject.toml.

pyproject.toml

[build-system]
requires = ["setuptools>=61.0", "matplotlib"]
build-backend = "setuptools.build_meta"

[project]
name = "mypkg"
etc...

[tool.setuptools.packages.find]
where = ["src"]

[tool.setuptools.package-data]
mypkg = ["ttf/*.ttf"]

What I get

When I use python -m build, I get

ModuleNotFoundError: No module named 'mypkg'

When I use python setup.py sdist and python setup.py bdist_wheel, the wheel is successfully build.
When installing the wheel with pip install dist/mypkg-[].whl, the install is successful and the matplotlib ttf library is populated with the custom fonts.

What I expect

I expect python -m build to work with importing mypkg post-install, like the setuptools cli does.