Improve typing of `inject`

[sobolevn]

globals() and locals() are both returning dict[str, Any]:

• https://github.com/python/typeshed/blob/712e4146ef255f14ecf396aac4996cdbef708312/stdlib/builtins.pyi#L1413
• https://github.com/python/typeshed/blob/712e4146ef255f14ecf396aac4996cdbef708312/stdlib/builtins.pyi#L1443

inject only add two protected attributes to a function / type. So, we can say that it returns the object unchanged: T -> T.