python: gpg.Context.decrypt verify_sigs and sink_result are bools

Both of these function-internal variables are never used for anything
other than a binary state. Implement them as the booleans they are.
Otherwise, casual readers of the code might think that they're
supposed to represent something other than a flag (e.g. "verify_sigs"
could mean "the signatures to verify", and "sink_result" could mean
"the place where we sink the result").

• Signed-Off-By: Daniel Kahn Gillmor <dkg@fifthhorseman.net>