In buildbot logs, when a worker process is run, two pipes are created for stdout and stderr. The problem is that using two pipes produce an output which is hard to read.

Pseudo output:
---
stdout time 1
stdout time 2
stdout time 3
stderr time 1
stderr time 2
---

I would prefer:
---
stdout time 1
stderr time 1
stdout time 2
stderr time 2
stdout time 3
---

Real logs:
https://buildbot.python.org/all/#/builders/79/builds/870
---
0:01:10 load avg: 4.48 [236/427/1] test_ftplib failed (env changed) -- (...)
Warning -- Uncaught thread exception: Exception
Exception in thread Thread-67:
Traceback (most recent call last):
  (...)
Exception
test__all__ (test.test_ftplib.MiscTestCase) ... ok
test_abort (test.test_ftplib.TestFTPClass) ... ok
test_acct (test.test_ftplib.TestFTPClass) ... ok
test_all_errors (test.test_ftplib.TestFTPClass) ... ok
(...)
Ran 94 tests in 2.326s
---

The Warning is logged at the beginning, it's not possible to know which test emitted this warning.

I propose to write stdout and stderr of a worker process into a single pipe to keep messages order.

In the past, I saw buildbot logging stderr messages in a different color. But it no longer seems to be the case.

I'm working on a PR.