refactor: mke Date class use __slots__

This should be the last slotting for a while.

Instrumenting roundup-admin and the Client class (running under the
waitress wsgi server) shows object count output similar to:

[(('IssueClass', 'roundup.backends.back_anydbm'), 128),
 (('Session', 'roundup.cgi.client'), 128),
 (('Mailer', 'roundup.mailer'), 128),
 (('Password', 'roundup.password'), 220),
 (('LiberalCookie', 'roundup.cgi.client'), 241),
 (('Database', 'roundup.backends.back_anydbm'), 256),
 (('FileClass', 'roundup.backends.back_anydbm'), 256),
 (('Number', 'roundup.hyperdb'), 256),
 (('PythonExpr', 'roundup.cgi.PageTemplates.PythonExpr'), 274),
 (('Proptree', 'roundup.hyperdb'), 276),
 (('Role', 'roundup.security'), 384),
 (('PathExpr', 'roundup.cgi.PageTemplates.Expressions'), 630),
 (('Class', 'roundup.backends.back_anydbm'), 640),
 (('SubPathExpr', 'roundup.cgi.PageTemplates.Expressions'), 645),
 (('Date', 'roundup.hyperdb'), 678),
 (('Link', 'roundup.hyperdb'), 934),
 (('Multilink', 'roundup.hyperdb'), 1024),
 (('Permission', 'roundup.security'), 6784),
 (('TruthDict', 'roundup.support'), 6795),
 (('PrioList', 'roundup.support'), 8192),
 (('Date', 'roundup.date'), 8610)]

where each row is a tuple of (class, module) and the count of
the number of object of that class sorted by number of objects.

I think the major classes that meet the criteria below are all
__slotted__ at this point:

  Is a native roundup class and not a subclass (e.g. LiberalCookie is
     subclass of http.cookies.SimpleCookie)
  Does not touch the database (not hyperdb or backend)
  Is not part of templating
  Is not a top level class that may need runtime attributes/methods
     overwritten e.g Session/Mailer/Password

Some of the excluded classes can be slotted, but they are not low
hanging fruit and requires more class heirarchy changes or more
extensive testing.

Author: rouilj

roundup/date.py

@@ -324,6 +324,9 @@ class Date:
         >>> test_fin(u)
     '''
 
+    __slots__ = ("year", "month", "day", "hour", "minute", "second",
+                 "_", "ngettext", "translator")
+    
     def __init__(self, spec='.', offset=0, add_granularity=False,
                  translator=i18n):
         """Construct a date given a specification and a time zone offset.