(🎁) multiple platform/version compatible mode

Feature

I want mypy to show errors for all platforms and versions, not just the current/selected one

import sys
if sys.version_info < (3, 11):
    print(21 + "sus")  # very funny, but is a type error 😢
if sys.platorm == "win32":
    print(21 + "sus")  # very funny, but is a type error 😢

actual:

> mypy test.py
no errors 😳

Instead I have to run mypy many times to get a comprehensive output:

> mypy --python-version 3.11 --platform lignux test.py
no errors 😳

> mypy --python-version 3.11 --platform darwin test.py
no errors 😳

> mypy --python-version 3.11 --platform win32 test.py
test.py:5: error: very funny, but that's a type error  [😳]
Found 1 error in 1 file (checked 1 source file)

> mypy --python-version 3.10 --platform lignux test.py
test.py:3: error: very funny, but that's a type error  [😳]
Found 1 error in 1 file (checked 1 source file)

> mypy --python-version 3.10 --platform win32 test.py
test.py:3: error: very funny, but that's a type error  [😳]
test.py:5: error: very funny, but that's a type error  [😳]
Found 2 errors in 1 file (checked 1 source file)

... etc, etc, etc ...

+ additional steps to curate the output into a readable format. (+ ratio?)

I would imagine that most libraries would want type checking applied for their supported python versions and platforms, and most python applications would probably want multiple platforms supported.

So almost everyone using mypy would want some level of cross support, and the current workaround includes manually building a solution involving running mypy concurrently and collecting / filtering the results. I would much rather this very common use-case be supported natively.

Perhaps some way of specifying a list of desired selections, and mypy could pass over the code with a matrix of those options. Perhaps mypy could run multiple processes to improve performance, or alternatively incorporate some complicated logic where it only does one run over the code but analyses the different paths/types correctly. The latter is the preferred solution in my mind, as it would be a lot more efficient and allow some additional benefits (#8823/#5940 comes to mind)

I would also expect mypy to show a note when an error is only detected under a diverging path, to make resolution easier.

pyproject.toml:

[tool.mypy]
platforms = ["lignux", "darwin", "win32"]
python_versions = [3.11, 3.10, 3.9, 3.8, 3.7]

> mypy test.py
test.py:3: error: very funny, but that's a type error  [😳]
test.py:3: note: error found with python-version < 3.11
test.py:5: error: very funny, but that's a type error  [😳]
test.py:5: note: error found with platform "win32"
Found 2 errors in 1 file (checked 1 source file)

Start with mypy's existing --python-version and --platform command-line options and the [tool.mypy] configuration examples in the issue. Define the supported version/platform matrix behavior and diagnostic aggregation before locating implementation and tests; done means one invocation reports errors from divergent paths without repeated manual runs.