Beta: raw_request_async with HTTPX

[richardm-stripe]

Summary

Adds async infrastructure and stripe.raw_request_async to the beta channel.

Details

Additions:

• stripe.raw_request_async - Gives a small surface area so that we can test async "end to end". We will codegen async methods for each API method in a follow-up.
• stripe.HTTPClientAsync - Defines the "interface" for defining an async http client.
• stripe.HTTPXClient - Default implementation of stripe.HTTPClientAsync. I picked httpx somewhat arbitrarily. It happens to be what openai used (a few months ago, openai-python was a fork of stripe-python with added async support, but this is no longer the case). We can add more or different defaults.
• stripe.default_http_client_async - A place for a user to set a custom async client
  (either a custom stripe.HTTPXClient or they can implement HTTPClientAsync.
• ApiRequestor.request_async, ApiRequestor.request_stream_async, ApiRequestor.request_raw_async - Infrastructure methods that lie between raw_request and http_client.

Notes:

• I omitted the ability to pass http_client_async into an ApiRequestor. You have to use the global http client. Mostly this is in the name of minimalism. Shouldn't be needed after StripeClient lands.
• The stripe.new_default_http_client_async function exists but it is not exported publicly. It doesn't make too much sense to add because there's only one baked-in HTTPClientAsync: HTTPX, so you might as well just construct a stripe.HTTPXClient.
• Since there are two http clients, users who want to use both sync and async interfaces might need to specify options such as verify_ssl_certs and proxy twice. In theory we could try and unify the interface for setting these settings on the global client, but I don't think this is an important enough use case to justify the additional complication.

[richardm-stripe]

Ok, this should be ready again. The git history got messy, but since you last looked I

• Added support for "proxy", "verify", and "timeout" to the HTTPX client
• Addressed code review
• Added preview overloads
• Added a lot of tests. This got tricky because the tests in test_http_client use MagicMock. For asynchronous functions you're supposed to use AsyncMock which exists in recent versions of our pytest-mock, but for python 3.6 and 3.7, we have to use an older version of pytest-mock which doesn't have AsyncMock, so I had to pull in another dependency.

[pakrym-stripe]

Some questions about http client tests