Improve typing for mocksvgconverter.py (#12788)

Co-authored-by: Adam Turner <9087854+AA-Turner@users.noreply.github.com>

Author: adamtheturtle

tests/roots/test-ext-imgmockconverter/mocksvgconverter.py

@@ -2,34 +2,33 @@
     Does foo.svg --> foo.pdf with no change to the file.
 """
 
+from __future__ import annotations
+
 import shutil
+from typing import TYPE_CHECKING
 
 from sphinx.transforms.post_transforms.images import ImageConverter
 
-if False:
-    # For type annotation
-    from typing import Any, Dict  # NoQA
+if TYPE_CHECKING:
+    from sphinx.application import Sphinx
+    from sphinx.util.typing import ExtensionMetadata
 
-    from sphinx.application import Sphinx  # NoQA
 
 class MyConverter(ImageConverter):
     conversion_rules = [
         ('image/svg+xml', 'application/pdf'),
     ]
 
-    def is_available(self):
-        # type: () -> bool
+    def is_available(self) -> bool:
         return True
 
-    def convert(self, _from, _to):
-        # type: (unicode, unicode) -> bool
+    def convert(self, _from: str, _to: str) -> bool:
         """Mock converts the image from SVG to PDF."""
         shutil.copyfile(_from, _to)
         return True
 
 
-def setup(app):
-    # type: (Sphinx) -> Dict[unicode, Any]
+def setup(app: Sphinx) -> ExtensionMetadata:
     app.add_post_transform(MyConverter)
 
     return {