tox packaging job fails in CI

E.g. as in https://travis-ci.org/github/matrix-org/python-signedjson/jobs/770347173

$ tox
GLOB sdist-make: /home/travis/build/matrix-org/python-signedjson/setup.py
packaging create: /home/travis/build/matrix-org/python-signedjson/.tox/packaging
packaging installdeps: check-manifest
packaging inst: /home/travis/build/matrix-org/python-signedjson/.tox/.tmp/package/1/signedjson-1.1.2.dev6+gde64b47.zip
packaging installed: canonicaljson==1.1.4,cffi==1.14.0,check-manifest==0.41,frozendict==1.2,importlib-metadata==1.6.0,pep517==0.8.2,pycparser==2.20,PyNaCl==1.3.0,signedjson==1.1.2.dev6+gde64b47,simplejson==3.17.0,six==1.14.0,toml==0.10.0,typing-extensions==3.7.4.2,unpaddedbase64==1.1.0,zipp==3.1.0
packaging run-test-pre: PYTHONHASHSEED='2381353217'
packaging run-test: commands[0] | check-manifest
lists of files in version control and sdist do not match!
missing from sdist:
  CHANGELOG.md
  RELEASING.md
  changelog.d
  pyproject.toml
suggested MANIFEST.in rules:
  include *.md
  include *.toml
ERROR: InvocationError for command /home/travis/build/matrix-org/python-signedjson/.tox/packaging/bin/check-manifest (exited with code 1)
___________________________________ summary ____________________________________
ERROR:   packaging: commands failed
The command "tox" exited with 1.
Done. Your build exited with 1.

Start by running tox and inspecting setup.py, the packaging job configuration, and the check-manifest output. Confirm why CHANGELOG.md, RELEASING.md, changelog.d, and pyproject.toml are absent from the source distribution, then verify that the packaging job passes with all required files included.