text: Title: Why is the version of FastAPI fixed in the install_requires?
  Body: Currently fastapi/pydantic versions are fixed in `setup.py`:
  ```
  requirements = [
      'fastapi==0.52.0',
      'pydantic==1.4',
      'contextvars;python_version<"3.7"'
  ]
  ```
  
  Is there a reason not to use `>=` instead?
  
  If compatibility is a concern, it's possible to test against different versions of FastAPI on TravisCI.