forward refs support added.

[dapper91]

fixes #60

This fix resolves the problem with postponed type declaration:

class TestModel(BaseXmlModel, tag='model1'):
    model1: 'TestSubModel' = element()

class TestSubModel(BaseXmlModel, tag='model2'):
    attr1: str = attr()

in suck case the type of the field model1 is ForwardRef and should be resolved after the TestSubModel model declared using update_forward_refs method.

TestModel.update_forward_refs()

[codecov-commenter]

Codecov Report

Merging #61 (22808c9) into dev (f6580e6) will decrease coverage by 0.66%.
The diff coverage is 87.17%.

❗ Your organization is not using the GitHub App Integration. As a result you may experience degraded service beginning May 15th. Please install the Github App Integration for your organization. Read more.

@@            Coverage Diff             @@
##              dev      #61      +/-   ##
==========================================
- Coverage   93.73%   93.08%   -0.66%     
==========================================
  Files          22       23       +1     
  Lines        1038     1085      +47     
==========================================
+ Hits          973     1010      +37     
- Misses         65       75      +10     

Flag	Coverage Δ
unittests	93.08% <87.17%> (-0.66%)	⬇️

Flags with carried forward coverage won't be shown. Click here to find out more.

Impacted Files	Coverage Δ
...ydantic_xml/serializers/factories/heterogeneous.py	91.07% <66.66%> (-3.58%)	⬇️
pydantic_xml/serializers/factories/homogeneous.py	91.37% <66.66%> (-3.45%)	⬇️
pydantic_xml/serializers/factories/wrapper.py	92.68% <66.66%> (-4.76%)	⬇️
pydantic_xml/serializers/factories/union.py	87.80% <71.42%> (-2.79%)	⬇️
pydantic_xml/serializers/factories/forwardref.py	90.47% <90.47%> (ø)
pydantic_xml/model.py	100.00% <100.00%> (ø)
pydantic_xml/serializers/factories/__init__.py	100.00% <100.00%> (ø)
pydantic_xml/serializers/factories/model.py	95.28% <100.00%> (+0.13%)	⬆️
pydantic_xml/serializers/serializer.py	97.47% <100.00%> (+0.44%)	⬆️