Fix libgpgme signing error due to Python2.7 buffer() interface changes.

See https://mail.python.org/pipermail/python-dev/2010-October/104917.html
    https://docs.python.org/2/c-api/buffer.html
    https://github.com/SoftwareIntrospectionLab/MininGit/issues/101#issuecomment-1602514

We have to test if our Python2.7 version has the new or old builtin
buffer() interface.
Merge branch 'hotfix/11522-gpg-signing' into develop