Fix fastAPI tests workflow

[dolfinus]

Change Summary

After tiangolo/fastapi#3712 CI workflow test fastAPI started constantly failing:
https://github.com/dolfinus/pydantic/runs/4766269380?check_suite_focus=true

============================= test session starts ==============================
platform linux -- Python 3.7.12, pytest-6.2.5, py-1.11.0, pluggy-1.0.0
rootdir: /home/runner/work/pydantic/pydantic/fastapi, configfile: pyproject.toml
plugins: anyio-3.4.0, sugar-0.9.4, hypothesis-6.31.6, cov-3.0.0, mock-3.6.1
collected 1244 items / 3 errors / 1241 selected

==================================== ERRORS ====================================
________ ERROR collecting docs_src/app_testing/app_b_py310/test_main.py ________
docs_src/app_testing/app_b_py310/test_main.py:3: in <module>
    from .main import app
docs_src/app_testing/app_b_py310/main.py:14: in <module>
    class Item(BaseModel):
docs_src/app_testing/app_b_py310/main.py:17: in Item
    description: str | None = None
E   TypeError: unsupported operand type(s) for |: 'type' and 'NoneType'
_ ERROR collecting docs_src/sql_databases/sql_app_py310/tests/test_sql_app.py __
docs_src/sql_databases/sql_app_py310/tests/test_sql_app.py:6: in <module>
    from ..main import app, get_db
docs_src/sql_databases/sql_app_py310/main.py:4: in <module>
    from . import crud, models, schemas
docs_src/sql_databases/sql_app_py310/crud.py:3: in <module>
    from . import models, schemas
docs_src/sql_databases/sql_app_py310/schemas.py:4: in <module>
    class ItemBase(BaseModel):
docs_src/sql_databases/sql_app_py310/schemas.py:6: in ItemBase
    description: str | None = None
E   TypeError: unsupported operand type(s) for |: 'type' and 'NoneType'
__ ERROR collecting docs_src/sql_databases/sql_app_py39/tests/test_sql_app.py __
docs_src/sql_databases/sql_app_py39/tests/test_sql_app.py:6: in <module>
    from ..main import app, get_db
docs_src/sql_databases/sql_app_py39/main.py:4: in <module>
    from . import crud, models, schemas
docs_src/sql_databases/sql_app_py39/crud.py:3: in <module>
    from . import models, schemas
docs_src/sql_databases/sql_app_py39/schemas.py:31: in <module>
    class User(UserBase):
docs_src/sql_databases/sql_app_py39/schemas.py:34: in User
    items: list[Item] = []
E   TypeError: 'type' object is not subscriptable
=========================== short test summary info ============================
ERROR docs_src/app_testing/app_b_py310/test_main.py - TypeError: unsupported ...
ERROR docs_src/sql_databases/sql_app_py310/tests/test_sql_app.py - TypeError:...
ERROR docs_src/sql_databases/sql_app_py39/tests/test_sql_app.py - TypeError: ...
!!!!!!!!!!!!!!!!!!! Interrupted: 3 errors during collection !!!!!!!!!!!!!!!!!!!!
============================== 3 errors in 4.68s ===============================

This is because list[Item] syntax used here is not valid in Python3.7 version used in the CI workflow.

So I've upgraded it to Python 3.10.

Related issue number

fix #3684

Checklist

• Unit tests for the changes exist
• Tests pass on CI and coverage remains at 100%
• Documentation reflects the changes where applicable
• changes/<pull request or issue id>-<github username>.md file added describing change
  (see changes/README.md for details)
• My PR is ready to review, please add a comment including the phrase "please review" to assign reviewers

[dolfinus]

please review

[aleksul]

fixes #3684

[dolfinus]

Currently CI/CD workflow is failing because of this issue: tiangolo/fastapi#4409

[PrettyWood]

I already fixed the CI in #3642 but opened #3690 to merge the fix directly.
Thanks though for the PR!