The builtin open() function accepts 'U' in the file mode, but it emits a DeprecationWarning since Python 3.4, change made in bpo-15204:

commit 6787a3806ee6a85a6f21ede70c10e15a6df267c4
Author: Serhiy Storchaka <storchaka@gmail.com>
Date:   Sat Nov 23 22:12:06 2013 +0200

    Issue #15204: Deprecated the 'U' mode in file-like objects.

$ cat x.py 
f = open(__file__, 'U')
line = f.readline()
print(type(line))
f.close()

$ python3.7 x.py 
x.py:1: DeprecationWarning: 'U' mode is deprecated
  f = open(__file__, 'U')
<class 'str'>

The 'U' mode is documented as deprecated since Python 3.3

The flag is basically only there for backward compatibility with Python 2. Python 3.9.0 final release is currently scheduled at 2020-06-08: 6 months after Python 2 end-of-life (2020-01-01).

Currently, the 'U' mode is documented as it will be removed from Python 4.0. But I dislike having a bunch of backward incompatible changes in Python 4.0. I would prefer Python 4.0 to be simply the version after Python 3.X.

Deprecation warnings are now displayed in the __main__ module by default, thanks to the PEP 565 -- Show DeprecationWarning in __main__.
https://www.python.org/dev/peps/pep-0565/

More and more projects run their test suite using -Werror and so have to fix deprecation warnings.

I prefer to push backward incompatible changes at the beginning of a dev cycle (Python 3.9), so we have room to revert it later if it causes too much trouble.

I reported the issue to docutils. In fact, docutils FileInput class was already fixed to prevent the deprecation warning, but there was no release yet:
https://sourceforge.net/p/docutils/bugs/363/

docutils is already fixed in the development version. I requested a release:
https://sourceforge.net/p/docutils/bugs/364/

I cited this change in the PEP 608 "Coordinated Python release".

New changeset e471e72977c83664f13d041c78549140c86c92de by Victor Stinner in branch 'master':
bpo-37330: open() no longer accept 'U' in file mode (GH-16959)
https://github.com/python/cpython/commit/e471e72977c83664f13d041c78549140c86c92de

Thanks for the review Serhiy ;-)

should we backport a documentation change for this? (the deprecatedremoved says 4.0 currently)

New changeset 1d2862a323ae1387e306942253b1e487018e2b7f by Victor Stinner in branch '3.8':
bpo-37330: open(): "U" mode is removed in Python 3.9 (GH-16972)
https://github.com/python/cpython/commit/1d2862a323ae1387e306942253b1e487018e2b7f

New changeset 4cb08b6c0ae6989d169dd48c2b24087941f6d0b0 by Miss Skeleton (bot) in branch '3.7':
bpo-37330: open(): "U" mode is removed in Python 3.9 (GH-16972)
https://github.com/python/cpython/commit/4cb08b6c0ae6989d169dd48c2b24087941f6d0b0

> should we backport a documentation change for this? (the deprecatedremoved says 4.0 currently)

Done. Thanks for the reminder.