I based this change on the official PyPI documentation:
  https://docs.pypi.org/trusted-publishers/
as well as a blog post walking through an example of how to implement
trusted publishing for a Python project:
  https://pgjones.dev/blog/trusted-plublishing-2023/
The diff is fairly minimal. All it does is remove the password used to
authenticate to PyPI, which we don't need anymore, and add a permission
needed for trusted publishing to operate.

This commit will be accompanied by some configuration in the project
settings on PyPI to allow this specific workflow to act as a trusted
publisher. Effectively, we're telling PyPI that when this particular
workflow (`publish.yml`) contacts it with an OIDC token from Github,
then PyPI should accept that as valid authentication and return an API
token which can be used to actually upload a package.

Convert to trusted publishing

The script runtests.py contains an archive of a very old version of
pytest along with a custom mechanism to import it. That used to be
the standard method of running tests (so I hear, anyway), but now it's
no longer relevant because we use tox for dependency management in
testing. So I'm removing the script. Having a large opaque blob of data
is not the greatest thing :-)

This commit changes the deployment to PyPI to be dependent on successful
deployment to Test PyPI. Without this we would have to manually check
that the deployment to Test PyPI succeeded before deploying to PyPI, and
there's really no point in making that a manual process. (Or, a person
could just deploy to PyPI directly without even waiting for the Test
PyPI deployment to succeed and in that case there's no point in even
deploying to Test PyPI first.)

I was inspired to do this by looking at the release workflow for some
other project, but I forget which one it was.

Make pypi deployment dependent on test-pypi deployment

Remove obsolete test code in runtests.py

Since none of these files are meant to be run directly as scripts,
there's no need to have shebang lines in them, and in fact it could give
someone a misleading impression to have them there, so removing them
should reduce confusion.

Closes #47

pytest_localserver.smtp allows running itself as a module to start up
an SMTP server, which is implemented by a bit of driver code guarded by
an `if __name__ == "__main__"` block. Typical best practice is to put
that code in a `main()` function, which is what I'm doing in this
commit.

Python 2.4 is no longer anywhere close to supported so we can put
`except` and `finally` in the same `try` block now.

Remove /usr/bin/env lines

Move SMTP driver code into a function

Update changelog for the upcoming 0.8 release