This library has a serious bug, see #244. The short story is: A global change in the error handling of unicode decoding breaks Pythons MIME code and potentially other things. See the comments in issue #49.
The maintainer has, as stated in the README, not much time to spend on this project (not blaming). This issue tries to warn potential users as long as the issues persists.
An alternative might be the "gpg" package, maintained by GnuPG iteself. It is a binding to the GpgME library. The downside is, that this package has a low level C-like API for certain operations.
Edit: Btw, the claim in the README regarding the shell injection is no longer up to date. The python-gnupg package by vinaysajip seems to be actively maintained.
Edit2: See #246 (comment) for a blog post by @isislovecruft (written before this issue was opened).