Fix typing issue, pin mypy in CI

Author: smelc

.github/workflows/lint.yml

@@ -18,3 +18,4 @@ jobs:
       - uses: jpetrucciani/mypy-check@master
         with:
           path: '.'
+          requirements_file: "dev-requirements.txt"

dev-requirements.txt

@@ -1 +1,2 @@
 black==25.9.0
+mypy=1.18.2

exdown.py

@@ -109,7 +109,7 @@ def from_buffer(
 
             if focus and focus != syntax.strip():
                 continue
-            skip = parse_skip(previous_line) if previous_line else None
+            skip: Optional[list[int]] = parse_skip(previous_line) if previous_line else None
             if skip is None:
                 pass  # Do not skip
             elif skip == []: