Make Python API compatible with Python 2 & 3

Re-factor the Python 2 Livestatus library to work under both 2/3:

* Replace some py2-only idioms and fix some errors like trying to access
  `.keys()` on a list.

* Let all literal strings be unicode on both py2/3
  (`from __future__ import unicode_literals`). This way, the modules
  code internally works with unicode stings (`unicode` on py2, `str` on
  py3) and converts to byte strings when writing to socket.

* API methods take either unicode strings or byte (`str` on py2, `bytes`
  on py3) as input.

Signed-off-by: Aksel Sjögren <asjogren@itrsgroup.com>

api/python/__init__.py

@@ -1,9 +1,20 @@
-from livestatus import SingleSiteConnection, MultiSiteConnection, MKLivestatusException, MKLivestatusSocketError, MKLivestatusSocketClosed, MKLivestatusQueryError, MKLivestatusNotFoundError
+from __future__ import absolute_import
+from .livestatus import (
+    SingleSiteConnection,
+    MultiSiteConnection,
+    MKLivestatusException,
+    MKLivestatusSocketError,
+    MKLivestatusSocketClosed,
+    MKLivestatusQueryError,
+    MKLivestatusNotFoundError,
+)
+
 __all__ = [
-    'SingleSiteConnection',
-    'MultiSiteConnection',
-    'MKLivestatusException',
-    'MKLivestatusSocketError',
-    'MKLivestatusSocketClosed',
-    'MKLivestatusQueryError',
-    'MKLivestatusNotFoundError']
+    "SingleSiteConnection",
+    "MultiSiteConnection",
+    "MKLivestatusException",
+    "MKLivestatusSocketError",
+    "MKLivestatusSocketClosed",
+    "MKLivestatusQueryError",
+    "MKLivestatusNotFoundError",
+]