Disable CI matrix for inner versions

[ion-elgreco]

Initial Checks

• I confirm that I'm using the latest version of Pydantic AI
• I confirm that I searched for my issue in https://github.com/pydantic/pydantic-ai/issues before opening this issue

Description

There is no need to run CI checks for Python versions that are above and below bounds. So we can disable 3.11 and 3.12 matrix

Example Code

Python, Pydantic AI & LLM client version

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[DouweM]

@ion-elgreco I agree that seems like an easy optimization, but before we dropped 3.9 we had cases where we needed different implementations of some type/signature inspection utils for 3.9, 3.10, and 3.10+, and we've seen version-specific issues various times in the history of maintaining Pydantic, so out of an abundance of caution we prefer to do too much rather than typically-just-enough-but-occassionally-too-little testing.