builder: add retries to composer API calls

[ondrejbudai]

The status calls are sometimes failing on:
upstream connect error or disconnect/reset before headers. reset reason:
connection termination

Since all requests are going through the company proxy, I think that the
networking isn't working 100% reliably. This commit adds a retry mechanism
provided by the urllib3 library. It will retry on all networking issues and
also on some 5xx errors that makes sense to retry (like gateway failures).

A test is added that runs the compose waiting code against a mock server
that fails every second request. This is imho sufficient to mimick a flaky
networking.

[gicmo]

There are a bunch of unrelated whitespace and import reordering changes.

[ondrejbudai]

These IDEs are smart... Fixed, thanks for noticing it! :)

[gicmo]

LGTM

[teg]

How long (in seconds) will we retry for? In fedora Koji we may have outages of up to 30sec.

[gicmo]

How long (in seconds) will we retry for? In fedora Koji we may have outages of up to 30sec.

backoff_factor ([float](https://docs.python.org/3/library/functions.html#float)) –
A backoff factor to apply between attempts after the second try (most errors are resolved immediately by a second try without a delay). urllib3 will sleep for:

{backoff factor} * (2 ** ({number of total retries} - 1))

seconds. If the backoff_factor is 0.1, then [Retry.sleep()](https://urllib3.readthedocs.io/en/latest/reference/urllib3.util.html#urllib3.util.Retry.sleep) will sleep for [0.0s, 0.2s, 0.4s, …] between retries. It will never be longer than backoff_max.

By default, backoff is disabled (set to 0).

[teg]

Thanks, won't really make a difference for he firewall issue then.

[gicmo]

Could make it a config option. Or just increase it?