better processing for subcommands

Old versions cached full output to memory and only after that printed it
into log. This behaviour causes issues in case if operation stucks and
you would need to find the step at which it does. New check_output
method uses Popen directly and iterates over stdout lines

Also changed behaviour from merging stderr into stdout to separate
stderr logging.

Any other behaviour of the function must be the same.

Also changed GPG.key_import method to disable local signing since it
seems it is useless (and may break process in case if there is no
private key)

tox.ini

@@ -32,4 +32,4 @@ deps =
 	{[tox]dependencies}
 	-e .[tests]
 commands =
-	pytest
+	pytest {posargs}