Bump pydantic dep to 1.7

[nmuesch]

Bumps the pydantic version from 1.5 -> 1.7
Most notably, this introduces support for python 3.9

Prior to this change, all apigentools commands run via Python 3.9 would fail with:

  File "/usr/local/Cellar/python@3.9/3.9.0/Frameworks/Python.framework/Versions/3.9/lib/python3.9/typing.py", line 648, in __getattr__
    raise AttributeError(attr)
AttributeError: __args__