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logging.StreamHandler contains the following code:
 stream.write(msg)
 stream.write(self.terminator)
 stream.flush()
When sys.stderr (or whatever other stream) is unbuffered, this results in two system calls and allows log records from different processes to concatenate on the same line in the output stream (followed by multiple newlines). This issue is new in Python 3.7, as stdout and stderr became "truly unbuffered" (cf. #30404).
As a simple solution, I believe the following would fix the issue and also be backward compatible:
 stream.write(msg + self.terminator)
 stream.flush()

New changeset b7d62050e7d5fc208ae7673613da4f1f2bc565c4 by Vinay Sajip (Josh Snyder) in branch 'master':
bpo-35046: do only one system call per line (logging.StreamHandler) (GH-10042)
https://github.com/python/cpython/commit/b7d62050e7d5fc208ae7673613da4f1f2bc565c4

New changeset d730719b094cb006711b1cd546927b863c173b31 by Vinay Sajip (Miss Islington (bot)) in branch '3.7':
bpo-35046: do only one system call per line (logging.StreamHandler) (GH-10042) (GH-10050)
https://github.com/python/cpython/commit/d730719b094cb006711b1cd546927b863c173b31

This change breaks (probably unsupported) uses of the logging module (esp. in combination with structlog) where non-string objects are passed through the .format() and .emit() calls.
Previously it was possible to set the terminator attribute to None (and then handling that case in a custom 'stream' implementation), which now breaks with a TypeError due to the concatenation.
In case anyone else runs into this (however unlikely that is). A workaround is to set the .terminator to something like this:
 class ConcatenableNothing:
 def __radd__(self, other):
 return other