service.Issue: Remove "mapping type" methods

https://docs.python.org/3/library/stdtypes.html#mapping-types-dict

We're no longer relying on implicit `dict()` conversion and are rather
using the `get_taskwarrior_record()` method directly.

I assume the original motivation for this was convenience of use in
third-party applications which used bugwarrior as a library. As far as I
know no such applications are still in use.

In preparation for documenting and stabilizing the base class interfaces
(#791) I'd like to cut them back to a minimum. Maybe the mapping type
could be added back some day if there were demand for it, but it would
then need to conform to a standard of testing a documentation I plan to
implement.

bugwarrior/services/__init__.py

@@ -290,49 +290,6 @@ def refine_record(self, record):
                 record[field] = getattr(self, 'get_default_%s' % field)()
         return record
 
-    def __iter__(self):
-        record = self.get_taskwarrior_record()
-        yield from record.keys()
-
-    def keys(self):
-        return list(self.__iter__())
-
-    def iterkeys(self):
-        return self.__iter__()
-
-    def items(self):
-        record = self.get_taskwarrior_record()
-        return list(record.items())
-
-    def iteritems(self):
-        record = self.get_taskwarrior_record()
-        yield from record.items()
-
-    def update(self, *args):
-        raise AttributeError(
-            "You cannot set attributes on issues."
-        )
-
-    def get(self, attribute, default=None):
-        try:
-            return self[attribute]
-        except KeyError:
-            return default
-
-    def __getitem__(self, attribute):
-        record = self.get_taskwarrior_record()
-        return record[attribute]
-
-    def __setitem__(self, attribute, value):
-        raise AttributeError(
-            "You cannot set attributes on issues."
-        )
-
-    def __delitem__(self, attribute):
-        raise AttributeError(
-            "You cannot delete attributes from issues."
-        )
-
     @property
     def record(self):
         return self._foreign_record