Check to see if these packages support type hints OR stubs for mypy compatibility

### Determine this is the right repository

- [x] I determined this is the correct repository in which to report this feature request.

### Summary of the feature request

The following libraries do not have type hints and stubs and thus present a challenge when `mypy` performs type hinting checks in the monorepo. A series of comments were added to the pandas-gbq library highlighting this fact as well as `# type: ignore[error-code]` pragmas.

Update these libraries to have type hinting and remove the comments and ignore pragmas.

* `# db-dtypes does not have type hints nor stubs that mypy uses for type checking.`
* `# pydata-google-auth does not have type hints nor stubs that mypy uses for type checking.`
* `# google-auth-oauthlib does not have type hints nor stubs that mypy uses for type checking.`

### Desired code experience

_No response_

### Expected results

### API client name and version

_No response_

### Use case

_e.g. This feature would be useful in a scenario where I need to process large amounts of data efficiently._

### Additional context

_e.g. This feature would help improve the performance of my application._

從 pandas-gbq 程式碼庫開始，找出與 db-dtypes、pydata-google-auth 和 google-auth-oauthlib 相關的註解與 `# type: ignore[error-code]` pragma。檢查這三個函式庫對型別檢查或 stub 的支援；當這些函式庫支援型別檢查，且可以移除所指出的註解與 ignore pragma 時，即視為完成。

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