fix exclusion of tests in wheel distribution

[dhimmel]

We included the following to exclude tests from our distributions:

manubot/setup.cfg

Lines 71 to 73 in 0c2e5b2

	[options.packages.find]
	exclude =
	tests

But that doesn't actually work. It only works to exclude a top-level tests directory from being detected by find_packages.

[dhimmel]

Ran the following to make sure this is working as intended:

>>> from setuptools import find_packages
>>> find_packages()
['manubot', 'manubot.tests', 'manubot.process', 'manubot.cite', 'manubot.webpage', 'manubot.pandoc', 'manubot.process.tests', 'manubot.cite.tests', 'manubot.cite.curie']
>>> find_packages(exclude=["tests"])
['manubot', 'manubot.tests', 'manubot.process', 'manubot.cite', 'manubot.webpage', 'manubot.pandoc', 'manubot.process.tests', 'manubot.cite.tests', 'manubot.cite.curie']
>>> find_packages(exclude=["tests", "*.tests"])
['manubot', 'manubot.process', 'manubot.cite', 'manubot.webpage', 'manubot.pandoc', 'manubot.cite.curie']

# remove cached files
rm -rf build dist/ manubot.egg-info/
# build wheel
python -m build --wheel
# list files in wheel, ensure no tests
unzip -l dist/manubot-*.whl