`send_each` vs `send_all`
1. `send_each` sends one HTTP request to V1 Send endpoint for each message in the list. `send_all` sends only one HTTP request to V1 Batch Send endpoint to send all messages in the array.
2. `send_each` uses concurrent.futures.ThreadPoolExecutor to run and wait for all `request` calls to complete and construct a `BatchResponse`. An `request` call to V1 Send endpoint either completes with a success or throws an exception. So if an exception is thrown out, the exception will be caught in `send_each` and turned into a `SendResponse` with an exception. Therefore, unlike `send_all`, `send_each` does not always throw an exception for a total failure. It can also return a `BatchResponse` with only exceptions in it.

`send_each_for_multicast` calls `send_each` under the hood.

* Add integration tests for send_each and send_each_for_multicast

Add test_send_each, test_send_each_500 and test_send_each_for_multicast

* chore: Fix pypy tests (#694)

* chore(auth): Update Auth API to `v2` (#691)

* `v2beta1` -> `v2`

* Reverting auto formatting changes

* undo auto formatting

* Add release notes to project URLs in PyPI (#679)

It's useful to be able to navigate to the release notes easily
from the package index when upgrading.

"Release Notes" is a special keyword that will have the scroll icon
in the project page.

A random example:

* https://pypi.org/project/streamlit/
* https://github.com/streamlit/streamlit/blob/815a3ea6fa3e7f9099b479e8365bd3a5874ddc35/lib/setup.py#L111

Co-authored-by: Lahiru Maramba <llahiru@gmail.com>

---------

Co-authored-by: Lahiru Maramba <llahiru@gmail.com>
Co-authored-by: pragatimodi <110490169+pragatimodi@users.noreply.github.com>
Co-authored-by: Samuel Dion-Girardeau <samueldg@users.noreply.github.com>