Minor issue. Using the ',b', ',o' or ',x' raises the error

ValueError("Cannot specify ',' or '_' with 'x'.",)

(or equivalently for 'b' and 'o'). However, it is possible to use the format specs '_b', '_o' and '_x' in Python 3.6 due to PEP 515. 

The following test demonstrates this:

>>> i = 10000
>>> for base in 'box':
...     for sep in ',_':
...         try:
...             print(f'{i:{sep}{base}}')
...         except ValueError as err:
...             print(repr(err))

ValueError("Cannot specify ',' or '_' with 'b'.",)
1_1000_0110_1010_0000
ValueError("Cannot specify ',' or '_' with 'o'.",)
30_3240
ValueError("Cannot specify ',' or '_' with 'x'.",)
1_86a0

The formatting part of PEP 515 was implemented in #27080.  Chris Angelico's initial patch https://bugs.python.org/file44152/underscores_decimal_only.patch was, as the name says, for decimal only, and added "or '_' " to the error message.  The next patch added other bases but neglected to remove the obsoleted addition.

Chris, can you do a PR?  I don't think any new test is needed.  I would also be inclined to skip the news blurb, but since Eric should review and merge, it is his call.

I agree there's no need for tests, since we usually don't guarantee such messages going forward. Also, I think skipping the news blurb is reasonable for this change.

New changeset 28773ca7a7aa58a28e42a9eb0066acf71b5a8dc4 by Terry Jan Reedy (Dargor) in branch 'master':
bpo-31780: Fix incorrect error message for ',x', ',b', ',o' specs (#4002)
https://github.com/python/cpython/commit/28773ca7a7aa58a28e42a9eb0066acf71b5a8dc4

New changeset 59b5c139d2ae95b1d1da63f81c3d777932215533 by Terry Jan Reedy (Miss Islington (bot)) in branch '3.6':
[3.6] bpo-31780: Fix incorrect error message for ',x', ',b', ',o' specs (GH-4002) (#4004)
https://github.com/python/cpython/commit/59b5c139d2ae95b1d1da63f81c3d777932215533

Thanks for handling that Terry. I take it the PR isn't needed now?

I would have done it, but I'm on the way to the US (literally posting this from the airport), and hadn't gotten to looking at this in the interval.

Correct.  Fixed.