Add RetryingCaller and AsyncRetryingCaller

[hynek]

Implements #45

Please lmk if it does what you need @RonnyPfannschmidt.

[RonnyPfannschmidt]

Great 👍

[hynek]

On a second look, and before I release anything – wouldn't it make more sense if on would be the first argument to __call__?

I.e.:

rc = RetryingCaller()
rc(httpx.HTTPError, func, kw=42)

You could get current behavior with:

rc = partial(RetryingCaller(), httpx.HTTPError)

For all I care, I could add some syntactic sugar:

rc = RetryingCaller().on(httpx.HTTPError)

wdyt?

[ambv]

Are RetryingCaller instances supposed to be reused? To me it feels more natural to just call the damn function, without thinking about exceptions at the same time. IOW, the current design.

I like adding def on(...) on RetryingCaller, that lets you have both worlds, in case you want to customize an existing instance to respond to another exception. But actually calling an rc instance stays clean, without having to specify the exception every time.

[hynek]

I think there's definitely the case for reuse where you want the same retry parameters but different errors to retry on.

[hynek]

fixed/implemented in #57