Build specification should list ‘towncrier.templates’ as a package

[bignose-debian]

When building the package using a PEP 517 build tool such as ‘build’, Setuptools emits this deprecation warning:

SetuptoolsDeprecationWarning: Installing 'towncrier.templates' as data is deprecated, please list it in packages.
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Package would be ignored

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Python recognizes 'towncrier.templates' as an importable package,
but it is not listed in the packages configuration of setuptools.

'towncrier.templates' has been automatically added to the distribution only
because it may contain data files, but this behavior is likely to change
in future versions of setuptools (and therefore is considered deprecated).

Please make sure that 'towncrier.templates' is included as a package by using
the packages configuration field or the proper discovery methods
(for example by using find_namespace_packages(...)/find_namespace:
instead of find_packages(...)/find:).

You can read more about "package discovery" and "data files" on setuptools
documentation page.

Using Python 3.11, ‘build’ version 0.9.0, ‘setuptools’ version 65.6.3.