Fix str unpack type propagation, use dedicated error code

[hauntsaninja]

Resolves #13823. Supersedes #15511. See also #6406

To avail of this, try --disable-error-code str-unpack

[github-actions]

Diff from mypy_primer, showing the effect of this PR on open source code:

spark (https://github.com/apache/spark)
+ python/pyspark/core/context.py:330: error: Unpacking a string is disallowed  [str-unpack]
+ python/pyspark/core/context.py:330: note: Error code "str-unpack" not covered by "type: ignore" comment

aiohttp (https://github.com/aio-libs/aiohttp)
+ aiohttp/web_log.py:197: error: Unused "type: ignore" comment  [unused-ignore]
+ aiohttp/web_log.py:197:30: error: Unpacking a string is disallowed  [str-unpack]
+ aiohttp/web_log.py:197:30: note: Error code "str-unpack" not covered by "type: ignore" comment
+ aiohttp/web_log.py:197:30: note: See https://mypy.rtfd.io/en/stable/_refs.html#code-str-unpack for more info
+ aiohttp/web_log.py:198: error: Unused "type: ignore[has-type]" comment  [unused-ignore]
+ aiohttp/web_log.py:199: error: Unused "type: ignore[has-type]" comment  [unused-ignore]
+ aiohttp/web_log.py:200: error: Unused "type: ignore[has-type]" comment  [unused-ignore]

operator (https://github.com/canonical/operator)
- ops/framework.py:153: error: Unpacking a string is disallowed  [misc]
+ ops/framework.py:153: error: Unpacking a string is disallowed  [str-unpack]