bpo-30157: Fix csv.Sniffer.sniff() regex pattern

[jake-jake-jake]

Looks like there is an extra > in the regex pattern for this sort of csv line: # ,".*?"

[mention-bot]

@jake-jake-jake, thanks for your PR! By analyzing the history of the files in this pull request, we identified @rhettinger, @benjaminp and @birkenfeld to be potential reviewers.

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[vstinner]

Please open a bug report to describe the bug and link it to this PR: then add "bpo-xxx" the PR title.

[jake-jake-jake]

@Haypo Let me know if anything else needs adjusting.

[louisom]

Change title to something like bpo-30157: Fix csv sniffer regex patter would be more descriptive.

[serhiy-storchaka]

I'm surprized that such bug was not caught by tests. This means the lack of tests. Needed a new test for that case.

[serhiy-storchaka]

Needed a test and a Misc/NEWS entry.

[jake-jake-jake]

I added some tests for the regex patterns that should ensure that Sniffer._guess_quote_and_delimiter() matches patterns as expected.

[jake-jake-jake]

@serhiy-storchaka Let me know if I need to make any more amendments to the tests or news entry; the tests should properly identify when the patterns fail to match.

[serhiy-storchaka]

Sniffer._guess_quote_and_delimiter() is an implementation detail. Is it possible to write tests for these cases using only public API (Sniffer.sniff())?

[brettcannon]

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[serhiy-storchaka]

Recreated as #5601.