The Python 3.x test suite produces a new regression on macOS 10.13 under the new APFS filesystem when executing the test_undecodable_filename() test from Lib/test/test_httpservers.py. The error appears as...

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ERROR: test_undecodable_filename
(test.test_httpservers.SimpleHTTPServerTestCase)
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Traceback (most recent call last):
  File "/Users/howarth/Python-3.6.2/Lib/test/support/__init__.py",
line 601, in wrapper
    return func(*args, **kw)
  File "/Users/howarth/Python-3.6.2/Lib/test/test_httpservers.py",
line 380, in test_undecodable_filename
    with open(os.path.join(self.tempdir, filename), 'wb') as f:
OSError: [Errno 92] Illegal byte sequence:
'/var/folders/7g/1x2rsy3j40n1pydq931hzlkm0000gn/T/tmpomp1r36b/@test_58317_tmp\udce7w\udcf0.txt'

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Ran 59 tests in 3.207s

See https://developer.apple.com/library/content/documentation/FileManagement/Conceptual/APFS_Guide/FAQ/FAQ.html

"""
APFS accepts only valid UTF-8 encoded filenames for creation, and preserves both case and normalization of the filename on disk in all variants..

Some differences between how APFS and HFS+ handle filenames include the following:
...
        • APFS doesn’t allow files to be created with filenames that contain unassigned codepoints in the Unicode 9.0 standard, whereas HFS+ does.