Make VALID and INVALID covariant (#541)

By using `Sequence`, these become covariant. Previously, doing something like:
```
    VALID = [
        ValidTestCase("..."),
        ValidTestCase("..."),
    ]
```

would be a type error in `pyre`, because `pyre` would infer this to be a `list[ValidTestCase]` which is *not* a subclass of `list[str | ValidTestCase]`. By making these `Sequence`s (and thus immutable), we can avoid the requirement of having explicit type annotations everywhere for this.

Author: alanhdu

src/fixit/rule.py

@@ -76,10 +76,10 @@ class LintRule(BatchableCSTVisitor):
     __ https://peps.python.org/pep-0440/#version-specifiers
     """
 
-    VALID: ClassVar[List[Union[str, Valid]]]
+    VALID: ClassVar[Sequence[Union[str, Valid]]]
     "Test cases that should produce no errors/reports"
 
-    INVALID: ClassVar[List[Union[str, Invalid]]]
+    INVALID: ClassVar[Sequence[Union[str, Invalid]]]
     "Test cases that are expected to produce errors, with optional replacements"
 
     AUTOFIX = False  # set by __subclass_init__