Previously, PyYAML was hosted on BitBucket, so we had a mirror of the repo on GitHub. Now, official hosting has moved to GitHub, so we can point to the official repository instead. Thanks to Marco A. Feliu <firstname.lastname@example.org> for pointing this out. This patch also updates the install shim to update submodule URLs. To preserve the functionality of earlier Dotbot versions, we will need to maintain 'https://github.com/anishathalye/pyyaml'. Because old versions of the install shim used with new Dotbot versions will not update submodule URLs, we will need to keep the old repository in sync with the upstream repository as we upgrade PyYAML versions. This patch also upgrades the dependency to PyYAML 3.12.