bpo-31780: Fix incorrect message for format spec

[pablogsal]

As can be checked in the discussion for bpo-31780 there is an incorrect error message when using the ',b', ',o' or ',x' formatters:

>>> 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

This PR fixes this so now we obtain:

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

https://bugs.python.org/issue31780

[miss-islington]

Thanks @pablogsal for the PR, and @terryjreedy for merging it 🌮🎉.. I'm working now to backport this PR to: 3.6.
🐍🍒⛏🤖

[bedevere-bot]

GH-4004 is a backport of this pull request to the 3.6 branch.

[terryjreedy]

If there were a more complete name in your bugs.python.org profile, I would have used it.