PEP-563 breaks SerializationStrategy

1. SerializationStrategy example with DateTimeFormats works fine in py3.7 https://github.com/Fatal1ty/mashumaro#user-defined-classes

2. however introduction of PEP-563 via from __future__ import annotations https://www.python.org/dev/peps/pep-0563/ breaks usage of SerializationStrategy with error trace:

Traceback (most recent call last):
  File ".../src/verify/code/mashu/mashu_pep563.py", line 22, in <module>
    class DateTimeFormats(DataClassDictMixin):
  File "/usr/lib/python3.7/site-packages/mashumaro/serializer/base/dict.py", line 19, in __init_subclass__
    raise exc
  File "/usr/lib/python3.7/site-packages/mashumaro/serializer/base/dict.py", line 15, in __init_subclass__
    builder.add_to_dict()
  File "/usr/lib/python3.7/site-packages/mashumaro/serializer/base/metaprogramming.py", line 163, in add_to_dict
    for fname, ftype in self.fields.items():
  File "/usr/lib/python3.7/site-packages/mashumaro/serializer/base/metaprogramming.py", line 53, in fields
    for fname, ftype in typing.get_type_hints(self.cls).items():
  File "/usr/lib/python3.7/typing.py", line 973, in get_type_hints
    value = _eval_type(value, base_globals, localns)
  File "/usr/lib/python3.7/typing.py", line 260, in _eval_type
    return t._evaluate(globalns, localns)
  File "/usr/lib/python3.7/typing.py", line 466, in _evaluate
    is_argument=self.__forward_is_argument__)
  File "/usr/lib/python3.7/typing.py", line 139, in _type_check
    raise TypeError(f"{msg} Got {arg!r:.100}.")
TypeError: Forward references must evaluate to types. Got <__main__.FormattedDateTime object at 0x7faec3540f60>.

[Fatal1ty]

I changed the way howSerializationStrategy classes are used. @rominf you might be interested in reading README here and here. At the moment these changes are in the master branch but I will release 2.0 version soon since it’s not backward compatible.

[Fatal1ty]

The new changes are in 2.0 now.