Generate changelog for version 0.3

This was done by running `tox -e towncrier -- --version 0.3`.

I've generated the changelog for version 0.3 even though this commit is
based on v0.3.1 because the changes made between those two versions
didn't have a changelog fragment anyway (they should, but I forgot), and
ensuring eventual consistency of the changelog is more important than
getting the changelog entries in the right release. Ultimately I just
don't think it's important enough to make it worth doing the technically
proper thing of making post-releases for both v0.3 and v0.3.1.

Merge pull request #176 from diazona/fix-failing-release-tests/2/dev

Handle "optional" dependencies with no extra

Merge pull request #172 from diazona/exclude-files-from-sdist/1/dev

Exclude some irrelevant config files from sdist

Merge pull request #146 from diazona/disable-pytest-distribution-pack…

…age-test-py312/1/dev

Conditionally disable the pytest distribution package test

Merge pull request #146 from diazona/disable-pytest-distribution-pack…

…age-test-py312/1/dev

Conditionally disable the pytest distribution package test

Merge pull request #97 from diazona/update-changelog-for-0.1/1/dev

Add new changelog entry for description fix

Merge pull request #93 from diazona/feature/update-changelog-release-0.1

Issue 91: CHANGELOG.rst: Update for initial release