In 3.7.0b4 I see the following traceback:

>>> import os
>>> os.errno
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
AttributeError: module 'os' has no attribute 'errno'

This was not the case for Python 3.6:

>>> import os
>>> os.errno
<module 'errno' (built-in)>

os.errno might not have been documented, however there are projects out there that use it:

https://github.com/intel/bmap-tools/issues/34
https://bugzilla.redhat.com/show_bug.cgi?id=1583196

So I suggest the removal of os.errno is something worth documenting at https://docs.python.org/3.7/whatsnew/3.7.html

The root of the change is in https://github.com/python/cpython/pull/1269/files#diff-405b29928f2a3ae216e45afe9b5d0c60