Python-cryptobox

* Add `cbox_fingerprint_prekey`

* New error codes `CBOX_INIT_ERROR` and `CBOX_DEGENERATED_KEY`.
* Implementation improvements:
- C-strings are checked for trailing `\0` byte.
- C-strings are length restricted (4096 bytes for file paths,
1024 bytes for session IDs).
- Debug builds contain NULL pointer checks.

* Compatibility release for Android cross-compiling with 1.6 stable.

* Bugfix release.

* Bugfix release.

* Add `cbox_is_prekey`.
* Update dependencies.