Fix type hint for build_ast

`ast.parse` returns an `ast.Module` in its default mode, 'exec':

3ce5502675/stdlib/ast.pyi (L159-L167)

Therefore this type hint can be more specific.

src/flake8/processor.py

@@ -218,7 +218,7 @@ class FileProcessor:
             (previous_row, previous_column) = end
         return comments, logical, mapping
 
-    def build_ast(self) -> ast.AST:
+    def build_ast(self) -> ast.Module:
         """Build an abstract syntax tree from the list of lines."""
         return ast.parse("".join(self.lines))