fix: Add pdm config for building source dist.
This fixes the errors in the 1.3.13 release which caused some of the files related to running tests to go missing. This resulted in the release to be yanked from PyPI a day after the release.
This fixes the errors in the 1.3.13 release which caused some of the files related to running tests to go missing. This resulted in the release to be yanked from PyPI a day after the release.