Replace getsig.java with a pure python implementation

Special thanks to deki for helping out with the certificate encodings:
https://gitlab.com/snippets/1842

fixes #5 https://gitlab.com/fdroid/fdroidserver/issues/5
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Internal Server Error - Gitea: Git with a cup of tea
500 Internal Server Error

Gitea Version: 1.28.0+dev-477-g8b6ad49a5f