More expressive sys.platform gating

### Describe the Bug

```
import sys

assert sys.platform == "win32"

import winreg
winreg.OpenKeyEx(winreg.HKEY_LOCAL_MACHINE, key_path)
```
or
```
import sys

if sys.platform != "win32":
raise EnvironmentError("Couldn't locate HKEY_LOCAL_MACHINE because it's only available on Windows system")

import winreg
winreg.OpenKeyEx(winreg.HKEY_LOCAL_MACHINE, key_path)
```

This works with MyPy but doesn't work with Pyrefly.

### Sandbox Link

https://pyrefly.org/sandbox/?code=JYWwDg9gTgLgBAZwJ4IFCoIYIQU1olAOjABsMYAzaEOAXlrgCIB3YAOwGYAmR1OfuKlCR8rNlBwBzVGImTCAeTA42AaRxIAogA8AFLKmEAEqs0BNAPoAZBQGEAglYsBZe7aMBJAHKaANHABrDQswcgALAEpUIA

### (Only applicable for extension issues) IDE Information

_No response_