Add note about wrong error code in type: ignore

[JukkaL]

If we change the error code of a message, it can result in existing
"type: ignore" comments being ignored. If this seems to be the case,
add a note that hints about changing the ignored error code.

Give a more specific message for a set of special cased error codes
where we know the original error code. In other cases give a more
vague message.

[JelleZijlstra]

Would it make sense to just always print this note if we detect a # type: ignore for the wrong code? It will be difficult to keep the hardcoded dictionary up to date, and it will always be helpful to alert users that they may be using the wrong error code.

[JukkaL]

That's a good idea. I'll add a fallback where we give a less specific error message if the error code is not in the dictionary. I think that it's still useful to maintain the dictionary, even if it's on a best-effort basis.

[github-actions]

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

pydantic (https://github.com/samuelcolvin/pydantic)
+ pydantic/networks.py:268: note: Error code changed to literal-required; "type: ignore" comment may be out of date
+ pydantic/networks.py:313: note: Error code changed to literal-required; "type: ignore" comment may be out of date
+ pydantic/networks.py:314: note: Error code changed to literal-required; "type: ignore" comment may be out of date

ignite (https://github.com/pytorch/ignite)
+ ignite/contrib/handlers/tensorboard_logger.py:155: note: Error code "import" not covered by "type: ignore" comment

aioredis (https://github.com/aio-libs/aioredis)
+ aioredis/connection.py:1201: note: Error code changed to literal-required; "type: ignore" comment may be out of date
+ aioredis/connection.py:1205: note: Error code changed to literal-required; "type: ignore" comment may be out of date

spark (https://github.com/apache/spark)
+ python/pyspark/sql/pandas/types.py:238: note: Error code "import" not covered by "type: ignore" comment
+ python/pyspark/sql/pandas/conversion.py:565: note: Error code "import" not covered by "type: ignore" comment
+ python/pyspark/sql/pandas/functions.pyi:70: note: Error code "name-defined" not covered by "type: ignore" comment
+ python/pyspark/pandas/typedef/typehints.py:32: note: Error code "import" not covered by "type: ignore" comment
+ python/pyspark/pandas/utils.py:43: note: Error code "import" not covered by "type: ignore" comment
+ python/pyspark/pandas/indexing.py:27: note: Error code "import" not covered by "type: ignore" comment
+ python/pyspark/pandas/generic.py:41: note: Error code "import" not covered by "type: ignore" comment
+ python/pyspark/pandas/frame.py:54: note: Error code "import" not covered by "type: ignore" comment
+ python/pyspark/pandas/base.py:28: note: Error code "import" not covered by "type: ignore" comment
+ python/pyspark/pandas/data_type_ops/categorical_ops.py:23: note: Error code "import" not covered by "type: ignore" comment
+ python/pyspark/pandas/categorical.py:21: note: Error code "import" not covered by "type: ignore" comment
+ python/pyspark/pandas/series.py:50: note: Error code "import" not covered by "type: ignore" comment
+ python/pyspark/pandas/groupby.py:47: note: Error code "import" not covered by "type: ignore" comment
+ python/pyspark/pandas/groupby.py:50: note: Error code "import" not covered by "type: ignore" comment
+ python/pyspark/pandas/indexes/base.py:35: note: Error code "import" not covered by "type: ignore" comment
+ python/pyspark/pandas/indexes/timedelta.py:21: note: Error code "import" not covered by "type: ignore" comment
+ python/pyspark/pandas/indexes/numeric.py:20: note: Error code "import" not covered by "type: ignore" comment
+ python/pyspark/pandas/indexes/multi.py:22: note: Error code "import" not covered by "type: ignore" comment
+ python/pyspark/pandas/indexes/datetimes.py:22: note: Error code "import" not covered by "type: ignore" comment
+ python/pyspark/pandas/indexes/category.py:20: note: Error code "import" not covered by "type: ignore" comment
+ python/pyspark/pandas/namespace.py:44: note: Error code "import" not covered by "type: ignore" comment
+ python/pyspark/pandas/plot/matplotlib.py:26: note: Error code "import" not covered by "type: ignore" comment
+ python/pyspark/pandas/data_type_ops/num_ops.py:23: note: Error code "import" not covered by "type: ignore" comment
+ python/pyspark/testing/pandasutils.py:27: note: Error code "import" not covered by "type: ignore" comment

pytest (https://github.com/pytest-dev/pytest)
+ src/_pytest/mark/structures.py:405: note: Error code "no-overload-impl" not covered by "type: ignore" comment
+ src/_pytest/mark/structures.py:423: note: Error code "no-overload-impl" not covered by "type: ignore" comment

pip (https://github.com/pypa/pip)
+ src/pip/_internal/utils/logging.py:156: note: Error code "assignment" not covered by "type: ignore" comment

pylox (https://github.com/sco1/pylox)
+ pylox/ast_printer.py:17: note: Error code "arg-type" not covered by "type: ignore" comment

core (https://github.com/home-assistant/core)
+ homeassistant/helpers/entity_platform.py:481: note: Error code changed to literal-required; "type: ignore" comment may be out of date
+ homeassistant/components/co2signal/sensor.py:102: note: Error code changed to literal-required; "type: ignore" comment may be out of date