Add support for collections.abc in Python >=3.8

[ganto]

Fixes the following error with Python >=3.8:

[...]
python ./run_tests.py
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ERROR: test_exports_failed_render (datatypes.tests.test_entity.TestEntityNoMock)                                                                              
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Traceback (most recent call last):                                                                                                                            
  File "/var/tmp/portage/dev-python/reclass-1.7.0/work/reclass-1.7.0/reclass/datatypes/tests/test_entity.py", line 291, in test_exports_failed_render         
    node1_exports = Exports({'a': '${a}'}, SETTINGS, '')                                                                                                      
  File "/var/tmp/portage/dev-python/reclass-1.7.0/work/reclass-1.7.0/reclass/datatypes/exports.py", line 24, in __init__                                      
    super(Exports, self).__init__(mapping, settings, uri)                                                                                                     
  File "/var/tmp/portage/dev-python/reclass-1.7.0/work/reclass-1.7.0/reclass/datatypes/parameters.py", line 67, in __init__                                   
    self.merge(mapping)                                                                                                                                       
  File "/var/tmp/portage/dev-python/reclass-1.7.0/work/reclass-1.7.0/reclass/datatypes/parameters.py", line 234, in merge                                     
    wrapped = self._wrap_dict(other)                                                                                                                          
  File "/var/tmp/portage/dev-python/reclass-1.7.0/work/reclass-1.7.0/reclass/datatypes/parameters.py", line 124, in _wrap_dict                                
    d[k] = self._get_wrapped(k, v)                                                                                                                            
  File "/var/tmp/portage/dev-python/reclass-1.7.0/work/reclass-1.7.0/reclass/datatypes/parameters.py", line 110, in _get_wrapped                              
    return self._wrap_value(value)                                                                                                                            
  File "/var/tmp/portage/dev-python/reclass-1.7.0/work/reclass-1.7.0/reclass/datatypes/parameters.py", line 102, in _wrap_value                                                                                                                                                                                               
    return Value(value, self._settings, self._uri,                                                                                                            
  File "/var/tmp/portage/dev-python/reclass-1.7.0/work/reclass-1.7.0/reclass/values/value.py", line 31, in __init__                                           
    self._item = self._parser.parse(value, self._settings)                                                                                                    
  File "/var/tmp/portage/dev-python/reclass-1.7.0/work/reclass-1.7.0/reclass/values/parser.py", line 68, in parse                                             
    tokens = parsers.listify(tokens)                                                                                                                          
  File "/var/tmp/portage/dev-python/reclass-1.7.0/work/reclass-1.7.0/reclass/values/parser_funcs.py", line 52, in listify                                     
    if (isinstance(w, collections.Iterable) and                                                                                                               
AttributeError: module 'collections' has no attribute 'Iterable'
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