define minimal version of fastapi

[RafalSkolasinski]

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Currently there seem to be no version requirements on fastapi in the setup.py. In clean environment it gave me fastapi in version

poetry show fastapi                                                                                    (mlserver)   (⎈ kind-ansible|seldon)
name         : fastapi
version      : 0.1.17
description  : FastAPI framework, high performance, easy to learn, fast to code, ready for production

dependencies
 - pydantic >=0.17
 - starlette >=0.9.7

that lead to

Traceback (most recent call last):
  File "/home/rskolasinski/miniconda3/envs/mlserver/bin/mlserver", line 5, in <module>
    from mlserver.cli import main
  File "/home/rskolasinski/miniconda3/envs/mlserver/lib/python3.8/site-packages/mlserver/__init__.py", line 2, in <module>
    from .server import MLServer
  File "/home/rskolasinski/miniconda3/envs/mlserver/lib/python3.8/site-packages/mlserver/server.py", line 7, in <module>
    from .model import MLModel
  File "/home/rskolasinski/miniconda3/envs/mlserver/lib/python3.8/site-packages/mlserver/model.py", line 3, in <module>
    from .codecs import (
  File "/home/rskolasinski/miniconda3/envs/mlserver/lib/python3.8/site-packages/mlserver/codecs/__init__.py", line 1, in <module>
    from .numpy import NumpyCodec, NumpyRequestCodec
  File "/home/rskolasinski/miniconda3/envs/mlserver/lib/python3.8/site-packages/mlserver/codecs/numpy.py", line 8, in <module>
    from .utils import SingleInputRequestCodec, is_list_of, InputOrOutput
  File "/home/rskolasinski/miniconda3/envs/mlserver/lib/python3.8/site-packages/mlserver/codecs/utils.py", line 21, in <module>
    from .errors import CodecError
  File "/home/rskolasinski/miniconda3/envs/mlserver/lib/python3.8/site-packages/mlserver/codecs/errors.py", line 1, in <module>
    from ..errors import MLServerError
  File "/home/rskolasinski/miniconda3/envs/mlserver/lib/python3.8/site-packages/mlserver/errors.py", line 1, in <module>
    from fastapi import status
  File "/home/rskolasinski/miniconda3/envs/mlserver/lib/python3.8/site-packages/fastapi/__init__.py", line 5, in <module>
    from .applications import FastAPI
  File "/home/rskolasinski/miniconda3/envs/mlserver/lib/python3.8/site-packages/fastapi/applications.py", line 3, in <module>
    from fastapi import routing
  File "/home/rskolasinski/miniconda3/envs/mlserver/lib/python3.8/site-packages/fastapi/routing.py", line 6, in <module>
    from fastapi import params
  File "/home/rskolasinski/miniconda3/envs/mlserver/lib/python3.8/site-packages/fastapi/params.py", line 4, in <module>
    from pydantic import Schema
ImportError: cannot import name 'Schema' from 'pydantic' (/home/rskolasinski/miniconda3/envs/mlserver/lib/python3.8/site-packages/pydantic/__init__.cpython-38-x86_64-linux-gnu.so)

Upgrading to fastapi==0.79.1 removes the issue so it may be good to put some minimal version in setup.py