title: SqlModel datetime field is throwing error upon execution
body: <p>I am using SQLModel in python 3.8</p>
  <p>When I add my datetime field <code>created_at: datetime = Field(default_factory=utcnow(), nullable=False)</code></p>
  <p>I get this
  <strong>Error</strong></p>
  <pre><code>File &quot;./app/main.py&quot;, line 16, in &lt;module&gt;
      class Post(SQLModel, table=True):
    File &quot;/Users/markwardell/PycharmProjects/pythonProject/venv/lib/python3.8/site-packages/sqlmodel/main.py&quot;, line 277, in __new__
      new_cls = super().__new__(cls, name, bases, dict_used, **config_kwargs)
    File &quot;pydantic/main.py&quot;, line 204, in pydantic.main.ModelMetaclass.__new__
    File &quot;pydantic/fields.py&quot;, line 488, in pydantic.fields.ModelField.infer
    File &quot;pydantic/fields.py&quot;, line 419, in pydantic.fields.ModelField.__init__
    File &quot;pydantic/fields.py&quot;, line 539, in pydantic.fields.ModelField.prepare
    File &quot;pydantic/fields.py&quot;, line 801, in pydantic.fields.ModelField.populate_validators
    File &quot;pydantic/validators.py&quot;, line 718, in find_validators
  RuntimeError: error checking inheritance of &lt;module 'datetime' from '/Applications/Xcode.app/Contents/Developer/Library/Frameworks/Python3.framework/Versions/3.8/lib/python3.8/datetime.py'&gt; (type: module)
  </code></pre>
  <p>If I do not the add the <code>created_at</code> the table is created in PostgresSql as expected.
  <strong>Code</strong></p>
  <pre><code>import datetime
  from typing import Optional
  import utcnow as utcnow
  from fastapi import FastAPI
  from sqlalchemy import TIMESTAMP, text
  from sqlmodel import Field, Session, SQLModel, create_engine, select
  from app.database import SQLALCHEMY_DATABASE_URL
  
  class Hero(SQLModel, table=True):
      id: Optional[int] = Field(default=None, primary_key=True)
      name: str = Field(index=True)
      secret_name: str
      age: Optional[int] = Field(default=None, index=True)
  
  
  class Post(SQLModel, table=True):
      id: Optional[int] = Field(default=None, primary_key=True, nullable=False)
      title: str = Field(nullable=False)
      content: str = Field(nullable=False)
      published: bool = Field(default=True, nullable=False)
      created_at: datetime = Field(default_factory=utcnow(), nullable=False)
  
  engine = create_engine(SQLALCHEMY_DATABASE_URL)
  
  
  def create_db_and_tables():
      SQLModel.metadata.create_all(engine)
  
  
  app = FastAPI()
  
  
  @app.on_event(&quot;startup&quot;)
  def on_startup():
      create_db_and_tables()
  </code></pre>