feat: reachability recognizes platform.system()

[atakiar]

Description

As title. Closes #8166. See PRs #8461 and python/typeshed#5671 for context

Test Plan

Duplicated all existing sys.platform test cases (minus the one that tests for sys.platform.startswith) and added several to ensure a valid mapping of sys.platform to platform.system()

[atakiar]

It's worth calling out that we don't add platform.system() to options (see comments on #8461) and use its value directly. Instead, we map sys.platform to platform.system(). This mapping isn't perfect and mypy behavior is not exactly the same between the two. Consider the following:

[case testUnreachableAfterToplevelAssertImport2]
# flags: --platform lol
import foo
foo.bar()  # No error

[file foo.py]
import sys
assert sys.platform == 'lol' # mypy sees this as ALWAYS_TRUE
def bar() -> None: pass

[case testUnreachableAfterToplevelAssertImport4]
# flags: --platform lol
import foo
foo.bar()  # No error

[file foo.py]
import platform
assert platform.system() == 'lol' # mypy sees this as TRUTH_VALUE_UNKNOWN
def bar() -> None: pass

[Akuli]

Note for reviewers: Instead of hard-coding platform.system() and a mapping of values in mypy (this pr), we could change how equality checks with literals work, and it might help other type checkers too (python/typeshed#5671).

[github-actions]

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

urllib3 (https://github.com/urllib3/urllib3)
+ test/conftest.py:28: error: Unused "type: ignore" comment

pytest (https://github.com/pytest-dev/pytest.git)
+ src/_pytest/doctest.py:302: error: Statement is unreachable  [unreachable]