sphinxdoc: ignore directory '.pc'

Hi,

Quilt (a patch management system used e.g. in Debian) creates directory .pc which contains patched files. If .rst files are patched during the build process, these files end up in .pc as well. This causes sphinxdoc to process these files, as it recusively scans the sources for .rst files. I suggest to add .pc to the 'exclude_patterns' in conf.py:

exclude_patterns = ['_build', '.tox', 'eggs', '.pc']

Please let me know if you want me to create a PR for this.

PS: A similar PR was accepted for mailman-core.