#341: Fixes for type-parameters without arguments

[USSX-Hares]

Closes #341

Changes:

• Updated tests.entities and tests.test_collections to include alternative forms of the same collection types (like builtins.list ("builtin") and typing.List (~~"generic"~~ "unbound") in addition to the existing typing.List[TypeA])
• Added new internal helper class ~~core._NoArgs~~ utils._NoArgs and its singleton instance ~~core._NO_ARGS~~ utils._NO_ARGS
• Added new methods ~~core._extract_args()~~ utils._get_type_args() and ~~core._extract_type_parameter()~~ utils._get_type_arg_param() which return either tp.__args__ value or _NO_ARGS constant
• All "unsafe" usages of tp.__args__ replaced with the new methods
• Added TypeVar special case check for core._decode_dict_keys()
  • Note: This is a special case for Python 3.7 and Python 3.8. By some reason, "unbound" dicts are counted as having key type parameter to be TypeVar('KT')
• Added tp.__origin__ == None special case for utils._get_type_origin()
  • Note: This is a special case for Python 3.6 exclusively. By some reason, some "unbound" types (typing.Set, typing.Tuple, and typing.FrozenSet) are considered to have this field with None rather than generating an AttributeError
  • Same for tp.__args__ and dicts

Status

• Without the fix, new tests fail on all versions of Python, but fail differently (see report in the comments of #341)
• By now, there are no failures related to the missing tp.__args__:
  • [x] Python 3.6: ~~Decodes "unbound" (but not "builtin") sets, tuples, and frozensets as lists rather than required types~~
  • [x] Python 3.7-3.8: ~~Fails to decode "unbound" and "builtin" dicts by trying to instantiate uninstantiatable typing.Dict.~~ Was actually caused by the strange structure of dict class nesting in specifically these Python versions, see note above
  • [x] No longer fails on Python 3.9+
  • [x] (New) Python 3.6: ~~Unable to decode "unbound" typing.Dict and dict keys on Python 3.6 exclusively~~

[UberMeatShield]

This seems like a pretty solid thing to get into the project so it is working in 3.10 as well :)

[lidatong]

Yes I'm sorry I've been MIA... I'll take a look soon!

[cprasad1]

@lidatong do you have plans to release a new version with this patch in? It will be great if we can get this bug fixed

[rucas]

@lidatong any update on this? Would be great to have a new version released with these features.