feat(generator): Universalize constraints path and grpcio requirement in gapic-generator templates

[chalmerlowe]

This PR updates the Python GAPIC generator templates to support modern EOL Python support drops natively across the entire SDK.

Changes

• setup.py.j2: Bumps the baseline minimum grpcio dependency requirement to >= 1.59.0. This robust minimum requirement resolves the pytest-asyncio event loop regressions universally, removing the need for custom pytest-asyncio overrides.
• noxfile.py.j2: Replaced the hardcoded constraints path with a dynamic constraints file path resolver at runtime, ensuring that monorepos natively resolve to their version-specific constraints files (e.g., testing/constraints-3.10.txt).

[gemini-code-assist]

Code Review

This pull request updates the minimum version of grpcio to 1.59.0 in setup.py.j2 and conditionally sets the LOWER_BOUND_CONSTRAINTS_FILE path in noxfile.py.j2 based on the existence of a testing directory. A review comment suggests making the constraints file path resolution more robust by checking for the existence of the specific versioned constraints file directly instead of just checking for the testing directory.