Fix self-check when targeting Python 3.10 (#12020)

This should fix wheel builds. Our CI always targets Python 3.6
when doing a self check, which means that we won't catch issues
specific to more recent Python versions.

Author: JukkaL

mypy/fastparse.py

@@ -123,6 +123,7 @@ def ast3_parse(source: Union[str, bytes], filename: str, mode: str,
         MatchClass = ast3.MatchClass
         MatchAs = ast3.MatchAs
         MatchOr = ast3.MatchOr
+        AstNode = Union[ast3.expr, ast3.stmt, ast3.pattern, ast3.ExceptHandler]
     else:
         Match = Any
         MatchValue = Any
@@ -133,6 +134,7 @@ def ast3_parse(source: Union[str, bytes], filename: str, mode: str,
         MatchClass = Any
         MatchAs = Any
         MatchOr = Any
+        AstNode = Union[ast3.expr, ast3.stmt, ast3.ExceptHandler]
 except ImportError:
     try:
         from typed_ast import ast35  # type: ignore[attr-defined]  # noqa: F401
@@ -357,7 +359,7 @@ def visit(self, node: Optional[AST]) -> Any:
             self.visitor_cache[typeobj] = visitor
         return visitor(node)
 
-    def set_line(self, node: N, n: Union[ast3.expr, ast3.stmt, ast3.ExceptHandler]) -> N:
+    def set_line(self, node: N, n: AstNode) -> N:
         node.line = n.lineno
         node.column = n.col_offset
         node.end_line = getattr(n, "end_lineno", None) if isinstance(n, ast3.expr) else None