POST Wikidata queries from name_usage.py instead of a GET

[derek73]

Closes #680.

tools/name_usage.py sent its SPARQL query as a GET. The query's VALUES block holds one literal per spelling per label language (63 languages). Percent-encoded, a Cyrillic letter takes six bytes, so a seven-letter word like Младший made the URL longer than the endpoint accepts and got HTTP 431. Measured: the largest URL that succeeded was 6,909 bytes (Младши, countries query) and the smallest that failed was 7,198 (Младший, totals query, which is sent first). A four-spelling word such as McDonald written in Cyrillic would also have failed.

The query can't shrink: the comment at WIKIDATA_LANGUAGES explains that exact literals keep it on the label index, while a FILTER times out. So this PR changes the transport. _sparql now POSTs a form-encoded query= body declared charset=utf-8, which has no length limit.

• Test: test_sparql_sends_the_query_in_the_body_not_the_url captures the Request and asserts POST, a bare endpoint URL, and a body that decodes back to the query. It also checks that the response is read from what urlopen returns. It failed before the fix.
• Live positive control: non-Latin names return counts through the POST. Иван gives 17,119 given and Петров 964 family, and 김 also gives counts. So the server decodes the body as UTF-8, and the zeros for the seven words the issue names are genuine answers, not failures to match. The control is run in Cyrillic because a misdecoded body would still run and match nothing, which looks like a false zero. An ASCII control (shehu, 180 family) cannot show that.
• No release-log bullet: the tool isn't shipped.

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[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 99.03%. Comparing base (625bc7e) to head (54c6256).
⚠️ Report is 2 commits behind head on master.

Additional details and impacted files

@@           Coverage Diff           @@
##           master     #683   +/-   ##
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  Coverage   99.03%   99.03%           
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  Files          46       46           
  Lines        4541     4543    +2     
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+ Hits         4497     4499    +2     
  Misses         44       44           

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