Workaround broken API in translate-toolkit

It seems that non working setcontext is there by design, so let's use
the attributes directly.

See https://github.com/translate/translate/pull/3433Signed-off-by: Michal Čihař <michal@cihar.com>

weblate/trans/exporters.py

@@ -113,6 +113,8 @@ class BaseExporter(object):
             self.handle_plurals(unit.get_target_plurals())
         )
         output.setcontext(self.string_filter(unit.context))
+        if hasattr(output, 'msgctxt'):
+            output.msgctxt = [self.string_filter(unit.context)]
         for location in unit.location.split():
             if location:
                 output.addlocation(location)

weblate/trans/tests/test_exporters.py

@@ -125,5 +125,4 @@ class TBXExporterTest(PoExporterTest):
 
 class MoExporterTest(PoExporterTest):
     _class = MoExporter
-    # Not supported in current translate-toolkit, but PR submitted
-    _has_context = None
+    _has_context = True