Migrate to Pydantic v2

[hongquan]

Fixes #70

What changes:

• Work with Pydantic v2 (not work with Pydantic v1, due to drastic change in Pydantic API).
• Raise minimum Python to 3.10, to support Django 5.0

You may notice change in testing data, about how to define "required" field. Look here for how it changed.

[codecov]

Codecov Report

Attention: 7 lines in your changes are missing coverage. Please review.

Comparison is base (2ffbdf8) 94.82% compared to head (463c7dc) 94.07%.

Files	Patch %	Lines
pyngo/openapi.py	83.33%	4 Missing ⚠️
pyngo/querydict.py	88.00%	3 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main      #94      +/-   ##
==========================================
- Coverage   94.82%   94.07%   -0.76%     
==========================================
  Files           8        8              
  Lines         232      253      +21     
==========================================
+ Hits          220      238      +18     
- Misses         12       15       +3     

☔ View full report in Codecov by Sentry.
📢 Have feedback on the report? Share it here.