inline typeshed stubs

[MarcoGorelli]

closes #16

The stubs in typeshed do work, but there are advantages to having then inlined, as detailed in the official typing guides https://typing.python.org/en/latest/guides/libraries.html#how-to-provide-type-annotations

We recommend using the inline type annotations approach, since it has the following benefits:

• Typically requires the least effort to add and maintain
• Users don’t have to download additional packages
• Always remains consistent with the implementation
• Allows library authors to type check their own code
• Allows language servers to show users relevant details about the implementation, such as docstrings and default parameter values