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.

Written by the indexing model from the issue text.