collect stub files recursively when building stub-only package

[hoefling]

The reason for this is that setuptools doesn't support globbing in package_data (see pypa/setuptools#1806), so stub files in subpackages will be collected via neither *.pyi nor **/*.pyi.

Please verify the solution using pathlib is viable. In its current form, this PR would restrict building of wheels and installation of source dists on Python>=3.4. The python_requires specifier is not set, but since the current wheels have py3 tag, I assume the solution is fine - if not, I can rewrite it using os.walk or switch to globbing in MANIFEST.in.

[hoefling]

Also, isort>=5 now includes support for configs in pyproject.toml by default, no extras needed - bumped that in tox config.