Support concatenating lists with different contents

[mrkmndz]

I believe this type relaxation is sound, given that __add__ creates a new list, so should be equivalent to [*self, *x], which works like this.

[JelleZijlstra]

#3183 was the last attempt to do this, and previously we had #2404. Unfortunately it's been hard to get right.

[mrkmndz]

makes sense, I'll defer to #3183 then.

[github-actions]

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

mypy (https://github.com/python/mypy.git)
+ mypy/strconv.py:410: error: Unsupported operand types for + ("List[Any]" and "List[Union[str, Tuple[str, List[Any]]]]") [operator]+ mypy/test/testcmdline.py:59: error: Unsupported operand types for + ("List[str]" and "List[str]") [operator]+ mypyc/test/test_commandline.py:51: error: Unsupported operand types for + ("List[str]" and "List[str]") [operator]
ignite (https://github.com/pytorch/ignite)
+ ignite/contrib/handlers/time_profilers.py:257: error: unused 'type: ignore' comment
pydantic (https://github.com/samuelcolvin/pydantic.git)
+ pydantic/color.py:199: error: unused 'type: ignore' comment
graphql-core (https://github.com/graphql-python/graphql-core.git)
+ src/graphql/type/validate.py:387: error: Unsupported operand types for + ("List[NamedTypeNode]" and "List[NamedTypeNode]")+ src/graphql/validation/rules/overlapping_fields_can_be_merged.py:81: error: Unsupported operand types for + ("List[FieldNode]" and "List[FieldNode]")
pandas (https://github.com/pandas-dev/pandas.git)
+ pandas/core/computation/scope.py:335: error: unused 'type: ignore' comment+ pandas/core/computation/scope.py:336: error: unused 'type: ignore' comment+ pandas/io/common.py:513: error: unused 'type: ignore' comment+ pandas/io/stata.py:909: error: unused 'type: ignore' comment+ pandas/io/pytables.py:3445: error: unused 'type: ignore' comment+ pandas/tests/reshape/concat/test_append.py:195: error: unused 'type: ignore' comment+ pandas/tests/frame/indexing/test_setitem.py:231: error: unused 'type: ignore' comment
paasta (https://github.com/yelp/paasta.git)
+ paasta_tools/config_utils.py:72: error: Unsupported operand types for + ("List[str]" and "List[str]")
tornado (https://github.com/tornadoweb/tornado.git)
+ tornado/autoreload.py:238: error: Unsupported operand types for + ("List[str]" and "List[str]")

[hauntsaninja]

Looks like we still have the weird List[str] + List[str] issues (#3183 (comment)).

This is definitely one for #1339