Importing patito fails with pydantic < 1.9.1.

[StefanBRas]

Hi,

importing patito fails when using older versions of pydantic even though their are new enough according to the pyproject.toml requirements (pydantic = ">=1.7.0")

It fails with

>>> import patito
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
  File "/home/sras/dev/venv/lib/python3.9/site-packages/patito/__init__.py", line 7, in <module>
    from patito.pydantic import Field, Model
  File "/home/sras/dev/venv/lib/python3.9/site-packages/patito/pydantic.py", line 507, in <module>
    class Model(BaseModel, metaclass=ModelMetaclass):
  File "/home/sras/dev/venv/lib/python3.9/site-packages/patito/pydantic.py", line 80, in __init__
    cls.DataFrame = DataFrame._construct_dataframe_model_class(
  File "/home/sras/dev/venv/lib/python3.9/site-packages/patito/polars.py", line 152, in _construct_dataframe_model_class
    f"{model.schema()['title']}DataFrame",
  File "pydantic/main.py", line 647, in pydantic.main.BaseModel.schema
  File "pydantic/schema.py", line 185, in pydantic.schema.model_schema
  File "pydantic/schema.py", line 617, in pydantic.schema.model_process_schema
  File "pydantic/schema.py", line 658, in pydantic.schema.model_type_schema
  File "pydantic/schema.py", line 258, in pydantic.schema.field_schema
  File "pydantic/schema.py", line 563, in pydantic.schema.field_type_schema
  File "pydantic/schema.py", line 922, in pydantic.schema.field_singleton_schema
  File "/usr/lib/python3.9/abc.py", line 123, in __subclasscheck__
    return _abc_subclasscheck(cls, subclass)
TypeError: issubclass() arg 1 must be a class

I'm using papito version 0.4.4 and I confirmed it to break for pydantic versions 1.7.0, 1.8.0 and 1.9.0.

No errors when using pydantic version 1.9.1 or above :)