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.

(cherry picked from commit 3debed96a8e04894b0064e8075ffd8c1dd4ce10b)
From branch: hotfix/11522-gpg-signing
Backport reason: Part of the fix for #11664.
Signed-off-by: Isis Lovecruft <isis@torproject.org>

Conflicts:
	lib/bridgedb/crypto.py
Fix error in EmailServer due to Python2.7 buffer() interface changes.

 * FIXES #11664.
Merge branch 'fix/11664-email-buffer-interface'