gpg_signature: Should not be a tuple

[woutdenolf]

Seems like PyPi has dropped PGP support: pypi/warehouse#13707

twine upload --sign /tmp/deploy/ewoksserver/assets/* 

Uploading distributions to https://upload.pypi.org/legacy/
Signing ewoksserver-0.1.0rc0.tar.gz
Uploading ewoksserver-0.1.0rc0.tar.gz
100% ━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━ 35.4/35.4 kB • 00:00 • 61.0 MB/s
WARNING  Error during upload. Retry with the --verbose option for more details.                                                                                           
ERROR    HTTPError: 400 Bad Request from https://upload.pypi.org/legacy/                                                                                                  
         gpg_signature: Should not be a tuple. 

[dstufft]

This should be fixed by pypi/warehouse#13738 -- PyPI still won't support GPG signatures, but it should upload successfully still, and just be discarded.