gh-138621: Increase test coverage for csv.DictReader and csv.Sniffer

[JanEricNitschke]

Previously there were no tests for the DictReader fieldnames setter, the case where a StopIteration was encountered when trying to determine the fieldnames from the content or the case where Sniffer could not find a delimiter.

Previously:

Name         Stmts   Miss Branch BrPart  Cover   Missing
--------------------------------------------------------
Lib/csv.py     262     10     96      8    95%   107->109, 164-165, 171, 255, 305->307, 312->314, 316-317, 327, 431, 434->443, 474, 511
--------------------------------------------------

After:

Name         Stmts   Miss Branch BrPart  Cover   Missing
--------------------------------------------------------
Lib/csv.py     262      5     96      6    97%   107->109, 305->307, 312->314, 316-317, 327, 434->443, 474, 511
--------------------------------------------------------
TOTAL          262      5     96      6    97%

---

This is my first attempt at contributing to CPython. I read through the dev guide. The easy issues (at least the recent ones i checked) seem to all already be assigned to someone and so i want the Increase test coverage.

Currently just going through modules alphabetically to check where i can see some fairly easy targets for increasing coverage. If there are any larger or more important modules i would love to have a look at those too if you could point me to them before i get to them in alphabetical order.

• Issue: Increase test coverage in csv module. #138621

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[JanEricNitschke]

I had originally skipped adding a test for the "check > 20" line because i thought that might be an implementation detail we dont want to fix down, but after seeing this, i realized this is officially documented so i think it makes sense to add a test for it.

This now gets us to

Name         Stmts   Miss Branch BrPart  Cover   Missing
--------------------------------------------------------
Lib/csv.py     262      4     96      5    97%   107->109, 305->307, 312->314, 316-317, 327, 434->443, 511
--------------------------------------------------------
TOTAL          262      4     96      5    97%

[JanEricNitschke]

I think the test failure is unrelated?

==> Fetching make
==> Downloading https://ghcr.io/v2/homebrew/core/make/blobs/sha256:94377dc5a364da305c75fd7aa923a42897993de9edd1eb074428e13c3f2aaf93
curl: (56) The requested URL returned error: 502
Error: make: Failed to download resource "make"
Download failed: https://ghcr.io/v2/homebrew/core/make/blobs/sha256:94377dc5a364da305c75fd7aa923a42897993de9edd1eb074428e13c3f2aaf93

[picnixz]

Yes, don't worry