在代码库中添加更多类型注解

There is a long discussion in gh-17945 about adding type annotations to the sympy codebase, but that is more of a historical issue with many tangents. Some parts of the codebase now have type annotations, and this issue is about incrementally adding more type annotations. PRs are welcome, both large and small (it is better to start small), that add type annotations to parts of the codebase where it makes sense to do so (not necessarily anywhere in the codebase – see below). You can see related issues and PRs under the typing label: https://GitHub.com/sympy/sympy/issues?q=label%3A%22typing%22