Add type arguments to generic types

[RobertCraigie]

I use pyright to type check my code. In strict mode it will report an error if a type is unknown or partially unknown. A type is considered unknown if one or some of it's parts or not specified, e.g.

# a is unknown
def foo(a) -> None:
    ...

# b is partially unknown
def foo(b: dict) -> None:
    ...

# c is known
def foo(c: dict[str, str]) -> None:
    ...

In essence the reasoning behind these diagnostics is for documentation, performance and to ensure consistent behaviour between different tools. For more details see their documentation, Typing Guidance for Python Libraries.

I encountered these diagnostics because of the missing type arguments for the extensions argument in Client.get although there appears to be other places that this is also relevant. You can find all the occurrences of unknown types by installing pyright and running pyright --verifytypes httpx.

[relsunkaev]

Got a PR ready for this.