Add a test to check that no old msgid or symbol are used

[Pierre-Sassoulas]

Type of Changes

	Type
✓	🐛 Bug fix

Description

This is a work in progress, I still need to make an exhaustive list of what was removed previously which is the hard part. I'm not sure if it's exhaustive but I can't think of any more msgid we deleted. Then we need to decide what we do when two name clash like for Message id 'W1601' cannot have both 'apply-builtin' and 'using-f-string-in-unsupported-version' as symbolic name.

I think it's better to add a simple check and later on think about making the performance better for run time checking of the existence of a message in #5607.

Closes #5729

[coveralls]

Pull Request Test Coverage Report for Build 1902891175

• 7 of 7 (100.0%) changed or added relevant lines in 1 file are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage increased (+0.002%) to 93.997%

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Totals
Change from base Build 1902688047:	0.002%
Covered Lines:	14937
Relevant Lines:	15891

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💛 - Coveralls

[DanielNoord]

Hm, this is difficult. This is an important change, but this is technically breaking... Should it be in 3.0?

[Pierre-Sassoulas]

I think we need to fix this "fast" as we don't want to have a lot of content on the web with W1601 and W1602 meaning the new message. The breaking change is reasonable imo as it only breaks disable using msgid and not the symbol directly. (Also, the symbol should be preferred we even have a check for that so it's probably not affecting a lot of users ?)

[DanielNoord]

Yeah and I guess you could consider the previous change breaking. As in, we're reverting a previously unnoticed breaking change here.
Let's just go ahead with it.

[Pierre-Sassoulas]

The timeout for python 3.8 seems repeatable. Do we put change the value to 15 ? It could be because we're calculating the coverage for 3.8, but it seems we're doing

      - name: Upload coverage artifact
        uses: actions/upload-artifact@v2.3.1
        with:
          name: coverage-${{ matrix.python-version }}
          path: .coverage

for all python version. And the other interpreter all take less than 4mn.

[Pierre-Sassoulas]

Let me rebase on #5841 :)

[DanielNoord]

Small changes and questions. Rest looks good to go!