compat: Python 3.8 no longer tolerates embezzling __classcell__ attrib

While this is highly internal thing, it seems that whenever Python's
semantic analyzer hits __class__ or super(), it will arrange for
injecting said new (rather internal) attribute towards __new__()
material instantiation of the the class as defined within respective
metaclass.  Then, when either of these will get effectively called,
presence of said attribute is checked.  Until 3.8, this was just
warning-worth, but with 3.8, it becomes a hard error.

In our case, (least resistance) solution is to simply refrain from
using either of those built-in constructs -- in fact, we don't really
need them(!).  Also the all-time explanatory guard is added for a good
measure.

Note that this may not be hit in casual run of clufter, but it certainly
was the case in the test suite, where the already prepared objects are
asked once again and hence our harness intercepting such repeated
requests and avoiding thus unnecessary computations kicks in.

Error sample (./run-tests command_manager filter_manager):
ERROR:clufter.plugin_registry:Module load error:
  <_io.TextIOWrapper name='/clufter/formats/simpleconfig.py' mode='r' encoding='utf-8'>:
  __class__ not set defining 'simpleconfig' as <class 'clufter.formats.simpleconfig.simpleconfig'>.
  Was __classcell__ propagated to type.__new__?

References:
https://docs.python.org/3/whatsnew/3.6.html#pep-487-simpler-customization-of-class-creation
https://docs.python.org/3/whatsnew/3.8.html#changes-in-the-python-api
python/cpython@f5e7b19

Signed-off-by: Jan Pokorný <[email protected]>

format.py

@@ -505,7 +505,7 @@ def _construct(self, protocol, *args, **kwargs):
         assert isinstance(protocol, basestring), \
                "protocol has to be string for `{0}', not `{1}'" \
                .format(self.__class__.name, protocol)
-        super(SimpleFormat, self)._construct(protocol, *args, **kwargs)
+        Format._construct(self, protocol, *args, **kwargs)
 
     @Format.producing(BYTESTRING)
     def get_bytestring(self, *iodecl):

formats/simpleconfig.py

@@ -1,5 +1,5 @@
 # -*- coding: UTF-8 -*-
-# Copyright 2017 Red Hat, Inc.
+# Copyright 2019 Red Hat, Inc.
 # Part of clufter project
 # Licensed under GPLv2+ (a copy included | http://gnu.org/licenses/gpl-2.0.txt)
 """Structured configuration formats such as corosync.conf"""
@@ -81,7 +81,7 @@ def get_bytestring(self, *iodecl):
         }
         """
         # try to look (indirectly) if we have a file at hand first
-        ret = super(simpleconfig, self).get_bytestring(self.BYTESTRING)
+        ret = SimpleFormat.get_bytestring(self, self.BYTESTRING)
         if ret is not None:
             return ret
 

plugin_registry.py

@@ -1,5 +1,5 @@
 # -*- coding: UTF-8 -*-
-# Copyright 2017 Red Hat, Inc.
+# Copyright 2019 Red Hat, Inc.
 # Part of clufter project
 # Licensed under GPLv2+ (a copy included | http://gnu.org/licenses/gpl-2.0.txt)
 """Easy (at least for usage) plugin framework"""
@@ -104,6 +104,10 @@ def probe(registry, name, bases, attrs=None):
         assert '-' not in name, "name cannot contain a dash"
         dname = cli_decor(name)
         attrs = attrs or {}
+        # seems more reasonable than plying even higher meta magic game
+        assert  not(attrs) or '__classcell__' not in attrs, \
+                ("{0} plugin: refrain from '__class__'/'super' (Py3.8+ limit)"
+                ).format(name)
         try:
             ret = registry._plugins[dname]
             log.info("Probe `{0}' plugin under `{1}' registry: already tracked"